Diffraction element, objective lens unit, optical pickup, optical disc apparatus and design method for diffraction element

The first and second materials of the diffraction element are selected such that the first material's refraction indexes n1(λ1), n1(λ2) and n1(λ3) for the first, second and third wavelengths λ1, λ2 and λ3 and the second material's refraction indexes n2(λ1), n2(λ2) and n2(λ3) for the first, second and third wavelengths λ1, λ2 and λ3 satisfy one of the conditions (1) or (2).

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese Patent Application JP2006-098707 filed in the Japanese Patent Office on Mar. 31, 2006, the entire contents of which being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diffraction element, objective lens unit, optical pickup, optical disc apparatus and design method for a diffraction element, and is preferably applied to an optical disc device, for example.

2. Description of Related Art

In recent years, there is an optical disc device that supports a plurality of types of optical discs: “Blu-ray Disc (Registered Trademark)” (BD) along with well-known formats such as Compact Disc (CD) and Digital Versatile Disc (DVD).

The optical disc device chooses one of the following types of optical beam in accordance with the format of an optical disc inserted: approximately 780 nm wavelength of optical beam for CD, around 660 nm wavelength of optical beam for DVD or about 405 nm wavelength of optical beam for BD.

By the way, it is desirable that the optical disc device be equipped with an objective lens that supports the three types of wavelengths to be simplified and downsized: The objective lens is installed in an optical pickup that emits an optical beam to an optical disc.

However, different wavelengths of optical beams are used for CD, DVD and BD formats. In addition, their protection layers are different in thickness (or the distances from lower surfaces of the optical discs to their signal recording surfaces are different). Moreover, their numerical apertures for objective lens are different.

Accordingly, it is difficult to design the objective lens that supports the three types of wavelengths. It is difficult to obtain one with good characteristics because of lower transmission efficiency and aberration of optical beams and the like.

Therefore, correcting the aberration of optical beams is one way to cope with the above problem: a diffraction element that selectively diffracts particular wavelengths of optical beams may be used along with the objective lens.

For instance, there is an optical disc device equipped with a diffraction element that only diffracts an optical beam for CD while optical beams for DVD and BD are not diffracted: the diffraction element has two different refraction indexes of substances and a step-like diffraction pattern formed in between these substances (see Jpn. Pat. Laid-open Publication No. 2005-302270 (Pages 15-19 and FIG. 12), for example). The optical beams for CD, DVD and BD are also referred to as a “CD-type optical beam”, “DVD-type optical beam” and “BD-type optical beam”, respectively.

SUMMARY OF THE INVENTION

Generally, materials which the optical beams pass through have different refraction indexes for each wavelength. In addition, the refraction indexes vary according to the materials.

Accordingly, if a suitable material is selected in a process of designing the diffraction element, both the diffraction efficiency of the CD-type optical beam and the transmission efficiency of BD- and DVD-type optical beams may rise. This improves the characteristics of the diffraction element.

If one can select a material based on certain criteria, its refraction indexes for each wavelength may be determined and the diffraction and transmission efficiencies of the diffraction element may be calculated. However, the criteria for selecting materials are not clearly defined to have good characteristics and to improve the diffraction and transmission efficiencies of optical beams.

Accordingly, there is a high possibility that the material be selected inappropriately. It means that the diffraction element having good characteristics can hardly be designed.

The present invention has been made in view of the above points and is intended to provide a diffraction element, objective lens unit, optical pickup and optical disc apparatus with good characteristics and a design method that allows easy designing of a diffraction element with good characteristics.

In one aspect of the present invention, a design method for designing a diffraction element including a first and second layer attached to one another and a diffraction pattern in which each of step-like protruding portions located at predetermined intervals between the first and second layers includes s steps wherein the first and second layers are respectively made from a first and second material through which a first, second and third wavelength λ1, λ2and λ3of an optical beam passes, includes a selection step of selecting the first material and the second material such that the first material's refraction indexes n1(λ1), n1(λ2) and n1(λ3) for the first, second and third wavelengths λ1, λ2and λ3and the second material's refraction indexes n2(λ1), n2(λ2) and n2(λ3) for the first, second and third wavelengths λ1, λ2and λ3satisfy one of the conditions (1) or (2).

Accordingly, it is easily determined that, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1), the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. It is also determined that, when the condition (2) is satisfied, the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. This allows easy selection of materials suitable for the diffraction element.

In another aspect of the present invention, a diffraction element includes: a first and second layer attached to one another; and a diffraction pattern in which each of step-like protruding portions located at predetermined intervals between the first and second layers includes s steps, wherein: the first layer and second layer are respectively made from a first and second material through which a first, second and third wavelength λ1, λ2and λ3of an optical beam passes; and the first material and the second material are selected such that the first material's refraction indexes n1(λ1), n1(λ2) and n1(λ3) for the first, second and third wavelengths λ1, λ2and λ3and the second material's refraction indexes n2(λ1), n2(λ2) and n2(λ3) for the first, second and third wavelengths λ1, λ2and λ3satisfy one of the conditions (1) or (2).

Accordingly, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1) the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. On the other hand, when the condition (2) is satisfied the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1.

In another aspect of the present invention, an objective lens unit includes: a diffraction element including a first and second layer attached to one another and a diffraction pattern in which each of step-like protruding portions located at predetermined intervals between the first and second layers includes s steps, the first and second layers being respectively made from a first and second material through which a first, second and third wavelength λ1, λ2and λ3of an optical beam passes; and an objective lens collecting the first, second or third wavelength λ1, λ2or λ3of the optical beam from the diffraction element, wherein the first material and the second material are selected such that the first material's refraction indexes n1(λ1), n1(λ2) and n1(λ3) for the first, second and third wavelengths λ1, λ2and λ3and the second material's refraction indexes n2(λ1), n2(λ2) and n2(λ3) for the first, second and third wavelengths λ1, λ2and λ3satisfy one of the conditions (1) or (2).

Accordingly, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1) the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. On the other hand, when the condition (2) is satisfied the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. The objective lens therefore can collect the first, second or third wavelength λ1, λ2or λ3of the optical beam.

In another aspect of the present invention, an optical pickup includes: a first light source emitting a first wavelength λ1of an optical beam; a second light source emitting a second wavelength λ2of an optical beam; a third light source emitting a third wavelength λ3of an optical beam; a diffraction element including a first and second layer attached to one another and a diffraction pattern in which each of step-like protruding portions located at predetermined intervals between the first and second layers includes s steps, the first and second layers being respectively made from a first and second material through which the first, second and third wavelengths λ1, λ2and λ3of the optical beam passes; and an objective lens collecting the first, second or third wavelength λ1, λ2or λ3of the optical beam from the diffraction element, the objective lens being integral with the diffraction element, wherein the first material and the second material are selected such that the first material's refraction indexes n1(λ1), n1(λ2) and n1(λ3) for the first, second and third wavelengths λ1, λ2and λ3and the second material's refraction indexes n2(λ1), n2(λ2) and n2(λ3) for the first, second and third wavelengths λ1, λ2and λ3satisfy one of the conditions (1) or (2).

Accordingly, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1) the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. On the other hand, when the condition (2) is satisfied the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. The objective lens therefore can collect the first, second or third wavelength λ1, λ2or λ3of the optical beam.

In another aspect of the present invention, an optical disc apparatus includes an optical pickup emitting a first, second or third wavelength λ1, λ2or λ3of an optical beam to a first, second or third optical disc, the optical pickup including: a first light source emitting the first wavelength λ1of the optical beam for the first optical disc; a second light source emitting the second wavelength λ2of the optical beam for the second optical disc; a third light source emitting the third wavelength λ3of the optical beam for the third optical disc; a diffraction element including a first and second layer attached to one another and a diffraction pattern in which each of step-like protruding portions located at predetermined intervals between the first and second layers includes s steps, the first and second layers being respectively made from a first and second material through which the first, second and third wavelengths λ1, λ2and λ3of the optical beam passes; and an objective lens collecting the optical beam from the diffraction element to the first, second or third optical disc, the objective lens being integral with the diffraction element, wherein the first material and the second material are selected such that the first material's refraction indexes n1(λ1), n1(λ2) and n1(λ3) for the first, second and third wavelengths λ1, λ2and λ3and the second material's refraction indexes n2(λ1), n2(λ2) and n2(λ3) for the first, second and third wavelengths λ1, λ2and λ3satisfy one of the conditions (1) or (2).

Accordingly, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1) the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. On the other hand, when the condition (2) is satisfied the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. Thus, the first, second or third wavelength λ1, λ2or λ3of the optical beam is emitted to the first, second or third optical disc.

According to one aspect of the present invention, it is easily determined that, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1), the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. It is also determined that, when the condition (2) is satisfied, the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. This allows easy selection of materials suitable for the diffraction element. Thus, the design method allows easy design of the diffraction element with good characteristics.

According to another aspect of the present invention, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1) the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. On the other hand, when the condition (2) is satisfied the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. Thus, the diffraction element presents good characteristics.

According to another aspect of the present invention, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1) the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. On the other hand, when the condition (2) is satisfied the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. The objective lens therefore can collect the first, second or third wavelength λ1, λ2or λ3of the optical beam. Thus, the objective lens unit presents good characteristics.

According to another aspect of the present invention, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1) the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. On the other hand, when the condition (2) is satisfied the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. The objective lens therefore can collect the first, second or third wavelength λ1, λ2or λ3of the optical beam. Thus, the optical pickup presents good characteristics.

According to another aspect of the present invention, when the refraction indexes n1and n2of the first and second materials for each wavelength λ satisfy the condition (1) the diffraction element diffracts the third wavelength λ3without diffracting the first and second wavelengths λ1and λ2. On the other hand, when the condition (2) is satisfied the diffraction element diffracts the second and third wavelengths λ2and λ3without diffracting the first wavelength λ1. Thus, the first, second or third wavelength λ1, λ2or λ3of the optical beam is emitted to the first, second or third optical disc. Accordingly, the optical disc apparatus presents good characteristics.

The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designate by like reference numerals or characters.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(1) Configuration of Optical Disc Device

(1-1) Overall Configuration of Optical Disc Device

FIG. 1shows an optical disc device1that supports an optical disc100of CD, DVD and BD. The optical disc device1reproduces signals from the optical disc100.

A control section2takes overall control of the optical disc device1. After the optical disc100is inserted into the optical disc device1, the control section2controls, in response to a playback command or the like from external devices (not shown), a drive section3and a signal processing section4to reproduce information from the optical disc100.

The signal processing section4is controlled by the control section2. The signal processing section4controls an optical pickup7to emit an optical beam to the optical disc100from an objective lens unit9.

The drive section3under the control of the control section2controls a spindle motor5to rotate the optical disc100at appropriate speeds. The drive section3also controls a sled motor6to bring an optical pickup7in a direction of tracking or the radial direction of the optical disc100. The drive section3also controls a two-axis actuator8to bring an objective lens unit9in a direction of focusing or close to the optical disc100, or in a direction of tracking away from the optical disc100. In this manner, the optical beam is focused on a target track on the optical disc100.

The signal processing section4detects the reflection from the signal recording surface of the optical disc100, and produces a reproduction signal based on the detected result, and then supplies the reproduction signal to an external device (not shown) through the control section2.

The optical pickup7supports three types of wavelength when emitting the optical beam through the objective lens unit9; the wavelength of 780 nm of the optical beam for the CD-type optical disc100c; the wavelength of 650 nm of the optical beam for the DVD-type optical disc100d; and the wavelength of 405 nm of the optical beam for the BD-type optical disc100b.

When reproducing signals from the optical disc100, the optical disc device1chooses, in accordance with the type of the optical disc100, one of the above beams and then emits it to the optical disc100.

(1-2) Configuration of Optical Pickup

As shown inFIG. 2, the optical pickup7includes sources of the optical beams: a laser diode11to emit the optical beams of 780 and 650 nm wavelengths for the CD- and DVD-types, respectively; and a laser diode12to emit the optical beam of 405 nm wavelengths for the BD-type. The optical beam for CD will be also referred to as a “CD-type optical beam Lc” while the optical beam for DVD and BD will be also referred to as a “DVD-type optical beam Ld” and a “BD-type optical beam Lb”, respectively.

A coupling lens13changes the optical magnification of the optical beam from the laser diode11.

The optical beam of particular wavelengths is reflected on a reflection-transmission layer14A of a beam splitter14while the optical beam with other wavelengths passes through the reflection-transmission layer14A; the CD-type optical beam Lc of around 780 nm and the DVD-type optical beam Ld of about 650 nm are reflected on the reflection-transmission layer14A while the BD-type optical beam LD of about 405 nm passes through the reflection-transmission layer14A.

The optical beam with particular polarization angles is reflected on a polarization layer15A of a polarization beam splitter15while the optical beam of other polarization angles passes through the polarization layer15A; the incident optical beam from the beam splitter14passes through the polarization layer15A while the incident optical beam from a collimator lens16, whose polarization angles have been adjusted, is reflected on the polarization layer15A.

The collimator lens16collimates the divergent light, which is the incident optical beam from the polarization beam splitter15, and transforms the collimated optical beam from a raise mirror17into convergent light.

The horizontal optical beam from the collimator lens16is reflected on the raise mirror17and then travels in the vertical direction or a direction perpendicular to the optical disc100; the vertical optical beam from a quarter wavelength plate18is reflected on the raise mirror17and then travels in the horizontal direction.

As for a part of the optical beam, its phase is delayed by one quarter of a wavelength through the quarter wavelength plate18. This transforms the optical beam from the raise mirror17from linearly polarized light into circularly polarized light while it transforms the optical beam from the objective lens unit9from circularly polarized light into linearly polarized light.

As shown inFIG. 3where a part of the cutting surface of the objective lens unit9is illustrated, a plane disc-shaped diffraction element20is attached to the bottom of a mirror tube19. The objective lens21is placed between the top and middle areas of the mirror tube19; the objective lens21includes a disc-shaped section whose size is almost the same as the diffraction element20and a smaller-diameter spindle-shaped section which is formed on the under surface of the disc-shaped section.

The objective lens unit9transforms the collimated optical beam from the quarter wavelength plate18into convergent light through the diffraction element20and the objective lens21to bring it to a focal point on the optical disc100.

In the optical pickup7, the optical beam diverged on the signal recording surface of the optical disc100is collimated through the objective lens21and diffraction element20of the objective lens unit9. The optical beam is then transformed from circularly polarized light to linearly polarized light through the quarter wavelength plate18. The optical beam then travels in the horizontal direction to the polarization beam splitter15after being reflected on the raise mirror17. Before getting into the polarization beam splitter15, the optical beam is transformed from collimated light to convergent light through the collimator lens16.

In this case, the optical beam with particular polarization angles is reflected on the polarization layer15A of the polarization beam splitter15. After that, the optical beam gets into a conversion lens22.

The conversion lens22changes the optical magnification of the CD-type optical beam Lc, the DVD-type optical beam Ld and the BD-type optical beam Lb. A optical axis synthesis element23makes the optical axes of the CD-type optical beam Lc and DVD-type optical beam Ld from the laser diode11and that of the BD-type optical beam Lb from the laser diode12all together.

On the surface of a photodetector24that is designed to receive the optical beam from the optical axis synthesis element23via the conversion lens22, a plurality of detection cells in a predetermined shape is formed. The detection cells detect the optical beam and then photoelectric-convert it. The detection cells subsequently supply resultant detection signals to the signal processing section4(FIG. 1).

The signal processing section4performs a predetermined calculation process and other processes using the detection signals from the photodetector24(FIG. 2) to obtain reproduction RF signals, and then performs, based on the reproduction RF signals, predetermined decoding and demodulation processes and the like to produce reproduction signals.

In addition, the signal processing section4(FIG. 1) performs, using the detection signals from the photodetector24(FIG. 2), a predetermined calculation process and other processes to produce drive control signals such as trucking error signals and focus error signals, and then supplies the drive control signals to the control section2. As a result, the control section2performs, through the drive section3, control processes such as trucking and focus control to adjust the optical beam to have the optical beam focused on the target track of the optical disc100. In this manner, the reproduction signals are appropriately produced.

When the control section2(FIG. 1) determines, based on a predetermined disc type determination method, that the optical disc100is CD-type (100c), the control section2controls the laser diode11of the optical pickup7(FIG. 2) to emit the CD-type optical beam Lc, or divergent light, from the light emitting point11A to the beam splitter14via the coupling lens13.

The CD-type optical beam Lc is reflected on the reflection-transmission layer14A of the beam splitter14, and then passes through the polarization beam splitter15. The CD-type optical beam Lc is subsequently collimated by the collimator lens16, and then reflected on the raise mirror17to travel in the vertical direction. The CD-type optical beam Lc is subsequently converted by the quarter wavelength plate18from linearly polarized light into circularly polarized light, and then reaches the objective lens unit9.

The objective lens unit9converts, through the diffraction element20and the objective lens21, the CD-type optical beam Lc from the quarter wavelength plate18into convergent light, and leads it to the focus point on the signal recording surface of the CD-type optical disc100c.

The objective lens unit9subsequently collimates, through the objective lens21and the diffraction element20, the divergent CD-type optical beam Lc which is the reflection from the signal recording surface of the CD-type optical disc100c, and then leads it to the quarter wavelength plate18.

After that, in the optical pickup7, the CD-type optical beam Lc is converted by the quarter wavelength plate18from circularly polarized light to linearly polarized light, and then is reflected on the raise mirror18to travel in the horizontal direction. The CD-type optical beam Lc is subsequently converted by the collimator lens16from collimated light to convergent light, and then reflected on the polarization layer15A of the polarization beam splitter15. After that, the CD-type optical beam Lc passes through the conversion lens22and the optical axis synthesis element23to reach the photodetector24.

The detection cells of the photodetector24detect the CD-type optical beam Lc, and transmit the resultant detection signals to the signal processing section4(FIG. 1).

The signal processing section4produces, based on the detection signals, the reproduction RF signals, and then generates, based on the reproduction RF signals, the reproduction signals. On the other hand, the signal processing section4produces the drive control signals such as trucking error signals and focus error signals.

When the control section2(FIG. 1) determines, based on a predetermined disc type determination method, that the optical disc100is DVD-type (100d), the control section2controls the laser diode11of the optical pickup7(FIG. 2) to emit the DVD-type optical beam Ld, or divergent light, from the light emitting point11B to the beam splitter14via the coupling lens13.

In a similar way to that of the CD-type optical disc100c, the DVD-type optical beam Ld is reflected on or passes through the following components: the coupling lens13, the beam splitter14, the polarization beam splitter15, the collimator lens16, the raise mirror17and the quarter wavelength plate18. After that, the DVD-type optical beam Ld is converted into convergent light through the diffraction element20and objective lens21of the objective lens unit9, and then is focused on the signal recording surface of the DVD-type optical disc100d.

After that, in a similar way to that of the CD-type optical disc100c, the objective lens21and diffraction element20of the objective lens unit9collimate the divergent DVD-type optical beam Ld, which is the reflection from the signal recording surface of the DVD-type optical disc100d. The DVD-type optical beam Ld is subsequently reflected on or passes through the following components: the quarter wavelength plate18, the raise mirror17, the collimator lens16, the polarization beam splitter15, the conversion lens22and the optical axis synthesis element23. As a result, the DVD-type optical beam Ld reaches the photodetector24.

In a similar way to that of the CD-type optical disc100c, the detection cells of the photodetector24detect the DVD-type optical beam Ld, and transmit the resultant detection signals to the signal processing section4(FIG. 1).

The signal processing section4produces, based on the detection signals, the reproduction RF signals, and then generates, based on the reproduction RF signals, the reproduction signals. On the other hand, the signal processing section4produces the drive control signals such as trucking error signals and focus error signals.

When the control section2(FIG. 1) determines, based on a predetermined disc type determination method, that the optical disc100is BD-type (100b), the control section2controls the laser diode12of the optical pickup7(FIG. 2) to emit the BD-type optical beam Lb, or divergent light, from the light emitting point12A to the beam splitter14.

In this case, the BD-type optical beam Lb passes through the reflection-transmission layer14A of the beam splitter14, and goes into the polarization beam splitter15.

After that, in a similar way to that of the CD-type optical disc100c, the BD-type optical beam Lb is reflected on or passes through the following components: the polarization beam splitter15, the collimator lens16, the raise mirror17and the quarter wavelength plate18. After that, the BD-type optical beam Lb is converted into convergent light through the objective lens21of the objective lens unit9, and then is focused on the signal recording surface of the BD-type optical disc100b.

By the way, in this case, the objective lens unit9allows the BD-type optical beam Lb to pass through the diffraction element20. It means that the diffraction element20does not diffract the BD-type optical beam Lb (described later).

After that, in a similar way to that of the CD-type optical disc100c, the objective lens21of the objective lens unit9collimates the divergent BD-type optical beam Lb, which is the reflection from the signal recording surface of the BD-type optical disc100b. The BD-type optical beam Lb is subsequently reflected on or passes through the following components: the quarter wavelength plate18, the raise mirror17, the collimator lens16, the polarization beam splitter15, the conversion lens22and the optical axis synthesis element23. As a result, the BD-type optical beam Lb reaches the photodetector24.

In a similar way to that of the CD-type optical disc100c, the detection cells of the photodetector24detect the BD-type optical beam Lb, and transmit the resultant detection signals to the signal processing section4(FIG. 1).

The signal processing section4produces, based on the detection signals, the reproduction RF signals, and then generates, based on the reproduction RF signals, the reproduction signals. On the other hand, the signal processing section4produces the drive control signals such as trucking error signals and focus error signals.

In this manner, the optical pickup7supports the CD-type optical disc100c, the DVD-type optical disc100dand the BD-type optical disc100b: with the objective lens unit9, the CD-type optical beam Lc, the DVD-type optical beam Ld and the BD-type optical beam Lb are focused on the signal recording surface of the optical disc100appropriately, and their reflection are correctly detected by the photodetector24.

(1-3) Configuration of Objective Lens Unit

FIG. 4is an enlarged sectional view of the CD-type optical disc100c, the DVD-type optical disc100d, the BD-type optical disc100band the objective lens unit9.

By the way,FIG. 4does not illustrate the two-axis actuator8(FIG. 1) which is attached to the objective lens unit9.

As for CD-, DVD- and BD-types, the following are standardized for compatibility: the wavelengths of optical beam to read out information; numerical apertures for collecting the optical beam; and the thickness of the optical discs100between the lower surface and the signal recording surface, or the thickness of the cover layer.

In reality, the CD-type optical disc is standardized in the following manner: the wavelength is approximately 780 nm; numerical apertures are approximately 0.45; and the thick of the cover layer is 1.2 mm. The DVD-type optical disc is standardized in the following manner: the wavelength is approximately 650 nm; numerical apertures are approximately 0.65; and the thick of the cover layer is 0.6 mm. The BD-type optical disc is standardized in the following manner: the wavelength is approximately 405 nm; numerical apertures are approximately 0.85; and the thick of the cover layer is 0.1 mm. The wavelengths λ for CD-, DVD- and BD-types are also represented as λc, λd and λb, respectively.

In addition, as for the CD-type optical beam Lc, the DVD-type optical beam Ld and the BD-type optical beam Lb, their focal distances, the distances between the objective lens21and their focal points, are different due to the characteristics of the objective lens21.

Accordingly, in the optical disc device1, the two-axis actuator8(FIG. 1) adjusts the distance between the objective lens unit9and the optical disc100to have the optical beam focused on the signal recording surface of the optical discs: the two-axis actuator8appropriately positions the objective lens unit9with respect to the optical disc100fixed at predetermined position.

By the way, for ease of explanation,FIG. 4illustrates the optical discs100whose positions are being adjusted with respect to the fixed objective lens unit9, resulting in different distances between the objective lens9and each optical disc's lower surface. In addition,FIG. 4only illustrates the cover layers of the CD-type optical disc100c, DVD-type optical disc100dand BD-type optical disc100b.

Considering the relative intensity of the BD-type optical beam Lb, the numerical apertures for BD-type and the like, the objective lens21is mainly designed for the BD-type optical beam Lb rather than the CD-type optical beam Lc and the DVD-type optical beam Ld.

Accordingly, when the collimated BD-type optical beam Lb reaches the lower surface of the objective lens21of the objective lens unit9, the objective lens21converts this incident BD-type optical beam Lb into convergent light to have it focused on the signal recording surface of the BD-type optical disc100b.

However, the objective lens21is designed for the BD-type optical beam Lb as mentioned above: if the collimated CD-type optical beam Lc or DVD-type optical beam Ld gets into the objective lens21via its lower surface, it may cause an aberration while the objective lens21converts it into convergent light. As a result, the optical beam may not be focused on the signal recording surface of the optical disc100appropriately.

Accordingly, the diffraction element20of the objective lens unit9only diffracts the CD-type optical beam Lc and DVD-type optical beam Ld to supply them to the objective lens21as non-collimated light. On the other hand, as the collimated BD-type optical beam comes in, the diffraction element20supplies it to the objective lens21as collimated light.

As a matter of fact, on an upper layer section20A of the diffraction element20, a diffraction grating for CD (also referred to as “CD-type diffraction grating”) DGc, or hologram, is formed to diffract only the CD-type optical beam Lc, not the DVD-type optical beam Ld and the BD-type optical beam Lb. As shown inFIG. 4, the CD-type optical beam Lc is slightly diffracted outward by the CD-type diffraction grating DGc.

That is to say, the upper layer section20A of the diffraction element20allows the DVD-type optical beam Ld and the BD-type optical beam Lb to pass through it while selectively diffracting the CD-optical beam Lc. In other words, the upper layer section20A of the diffraction element20is designed to only correct the aberration for the CD-type optical beam Lc.

After that, as shown inFIG. 4, the CD-type optical beam Lc from the diffraction element20is refracted through the lower and upper surfaces of the objective lens21. This converts the CD-type optical beam Lc into convergent light. In this manner, the objective lens unit9corrects the aberration for the CD-type optical beam Lc, and leads the CD-type optical beam Lc from the objective lens21to a focal point on the signal recording surface of the CD-type optical disc100c.

In addition, on a lower layer section20B of the diffraction element20, a diffraction grating for DVD (also referred to as “DVD-type diffraction grating”) DGd, or hologram, is formed to diffract only the DVD-type optical beam Ld, not the CD-type optical beam Lc and the BD-type optical beam Lb. As shown inFIG. 4, the DVD-type optical beam Ld is slightly diffracted outward by the DVD-type diffraction grating DGd.

That is to say, the lower layer section20B of the diffraction element20allows the CD-type optical beam Lc and the BD-type optical beam Lb to pass through it while selectively diffracting the DVD-optical beam Ld. In other words, the lower layer section20B of the diffraction element20is designed to only correct the aberration for the DVD-type optical beam Ld.

After that, as shown inFIG. 4, the DVD-type optical beam Ld from the diffraction element20is refracted through the lower and upper surfaces of the objective lens21. This converts the DVD-type optical beam Ld into convergent light. In this manner, the objective lens unit9corrects the aberration for the DVD-type optical beam Ld, and leads the DVD-type optical beam Ld from the objective lens21to a focal point on the signal recording surface of the DVD-type optical disc100d.

In this manner, in the objective lens unit9, the upper layer section20A of the diffraction element20only corrects the aberration for the CD-type optical beam Lc by diffracting it while the lower layer section20B of the diffraction element20only corrects the aberration for the DVD-type optical beam Ld by diffracting it. That can appropriately lead the CD-type optical beam Lc, the DVD-type optical beam Ld or the BD-type optical beam Lb to focal points of the signal recording surface of the CD-type optical disc100c, the DVD-type optical disc100dor the BD-type optical disc100beven after they pass through the objective lens21designed for the BD-type optical beam Lb.

(1-4) Configuration of Diffraction Element

As shown inFIG. 5A, the diffraction element20includes a flat, disc-shaped base layer20C. Its upper layer section20A includes the CD-type diffraction grating DGc while its lower layer section20B includes the DVD-type diffraction grating DGd, as mentioned above.

The base layer20C is for example made from transparent synthetic resin with a predetermined refractive index. Its interface to air or other materials can diffract the optical beam.

FIG. 5Bis an enlarged sectional view of the upper layer section20A. The CD-type diffraction pattern PTc is formed on an upper surface of the base layer20C: the CD-type diffraction pattern PTc includes a plurality of step-like protruding parts located at certain intervals. The CD-type diffraction pattern PTc is covered by a cover layer20D that is for example made from transparent resin.

The step-like CD-type diffraction pattern PTc includes three steps for each protruding part: the height of the protruding parts from bottom to top is 12 μm; and the interval of protruding parts, or the distance between one protruding part to the adjoining protruding part, is 18 μm. As shown inFIG. 3, the CD-type diffraction pattern PTc is concentrically formed on the upper surface of the diffraction element20within one-half radius from the center.

The cover layer20D is made from a transparent material whose refraction index is different from that of the base layer20C. A lower surface of the cover layer20D is attached to the CD-type diffraction pattern PTc without no space between them. An upper surface of the cover layer20D is substantially flat.

In this manner, the upper layer section20A of the diffraction element20includes the step-like CD-type diffraction pattern PTc whose protruding portions are located at certain intervals on the upper surface of the base layer20C. On the CD-type diffraction pattern PTc, the cover layer20D is formed: the refraction index of the cover layer20D is different from that of the base layer20C. Accordingly, the upper layer section20A diffracts the optical beam of particular wavelengths while the optical beam of other wavelengths passes through it without being diffracted. In this case, the CD-type diffraction grating DGc only diffracts the CD-type optical beam Lc.

FIG. 5Cis an enlarged sectional view of the lower layer section20B. On a flat lower surface of the base layer20C, a diffraction pattern layer20E including a DVD-type diffraction grating DGd is formed.

The diffraction pattern layer20E is made from a transparent resin whose refraction index is substantially the same as that of the base layer20C. The lower surface of the diffraction pattern layer20E has a step-like DVD-type diffraction pattern PTd whose protruding portions are located at certain intervals. The lower surface of the diffraction pattern layer20E is an interface to air because it is not covered with any materials.

The step-like DVD-type diffraction pattern PTd includes five steps for each protruding part: the height of the protruding parts from bottom to top is 6 μm; and the interval of protruding parts, or the distance between one protruding part to the adjoining protruding part, is 170 μm. As shown inFIG. 3, the DVD-type diffraction pattern PTd is concentrically formed on the lower surface of the diffraction element20within two-thirds radius from the center.

In this manner, the lower layer section20B of the diffraction element20includes the step-like DVD-type diffraction pattern PTd on the diffraction pattern layer20E attached to the lower surface of the base layer20C: the DVD-type diffraction pattern PTd includes protruding parts located at certain intervals. That diffracts particular wavelengths of optical beams while other wavelengths pass through it without being diffracted. That serves as a DVD-type diffraction grating DGd to only diffract the DVD-type optical beam Ld.

(2) Designing Diffraction Elements

The method for designing the diffraction element20will be described. WithFIGS. 5B and 6, the following describes a condition of selecting materials for the diffraction element20. For ease of explanation, the base and cover layers20C and20D inFIG. 6are illustrated in the following manner: the base layer20C is represented as a first layer made from a material M1; and the cover layer20D is represented as a second layer made from a material M2. The diffraction grating DG contains the diffraction pattern PT whose step-like protruding parts are located at certain intervals in between the two layers.

In this case, the number of steps of one protruding portion of the diffraction pattern PT is represented as “s” while the height of the protruding portion is represented as “d”.

In addition, the optical beam L passes through the first layer (or the material M1) and the second layer (or the material M2) in that order.

(2-1) Difference of Optical Path for One Wavelength

An optical path of a certain wavelength λ will be described. The refraction index of the material M1for the wavelength λ is represented as “n1” while the refraction index of the material M2for the wavelength λ is represented as “n2”.

When the optical beam L1passes through an area AR1containing one protruding portion, it means that the optical beam L1goes through the material M1. In this case, the length of the optical path is represented as (n1×d). When the optical beam L2passes through an area AR1containing one protruding portion, it means that the optical beam L2goes through the material M2. In this case, the length of the optical path is represented as (n2×d).

As a result, the optical path difference between the optical beams L1and L2is represented as (n2−n1)d in the area AR1. As indicated by an equation (4) below, a phase difference φ is calculated by dividing the optical path difference (n2−n1) d by the wavelength λ and multiplying the result of the division by 2π.

The diffraction process of the diffraction grating DG will be described. If the phase difference φ1 of one protruding portion of the diffraction grating DG is substantially equal to the wavelength λ multiplied by the a whole number, the optical beam L with the wavelength λ will not be affected by the diffraction pattern PT. It means that the optical beam L with the wavelength λ will not be diffracted by the diffraction pattern PT. This point is represented as follows:
φ1=2π·p(5)
where “p” is any whole number. The fact that “the diffraction grating DG does not diffract the wavelength λ because of satisfying the equations (4)-(5)” is also referred to as “this is zero-order type about the wavelength λ”.

By contrast, when the optical beam L with the wavelength λ is diffracted by the diffraction grating DG, the above equation (5) is not satisfied. To maximize the diffraction efficiency of the diffraction grating DG, the adjacent phases may need to be smoothly connected with one another between the adjacent protruding portions of the diffraction pattern PT, resulting in no difference between the phases.

In other words, if there are more steps (more than s steps) at one protruding portion as indicated by the dotted lines inFIG. 6, the phase difference φ1 for the s steps may need to be equal to the wavelength λ multiplied by a whole number. This point is represented as follows:

ϕ⁢⁢s=2⁢π⁢s·(n⁢⁢2⁢(λ)-n⁢⁢1⁢(λ))·dλ(6)ϕ⁢⁢s=2⁢π·q(7)
where “q” is any whole number. If the whole number q can be divided by the number of steps s, the optical beam L with the wavelength λ may not be diffracted because the phase difference φ for one step of the protruding portion is substantially equal to the wavelength λ multiplied by a whole number. Accordingly, in order to have the wavelength λ of the optical beam L diffracted at the diffraction grating DG, “s” (or the number of steps) and “q” may need to be prime to each other. This point is represented as follows:
gcd(s,q)=1  (8)
In this case, gcd(a,b) for example represents the greatest common divisor of an integer “a” and an integer “b”.

The fact that “the diffraction grating DG diffracts the wavelength λ because of satisfying the equations (6)-(8)” is also referred to as “this is first-order type about the wavelength λ”.

Accordingly, the following is evident within the area including one protruding portion of the diffraction pattern PT: Considering path differences of the wavelength λ of the optical beam L passing through the diffraction pattern PT, the equations (4)-(5) are satisfied to be the zero-order type while the equations (6)-(8) are satisfied to be the first-order type.

By the way, as the number of steps “s” of the first-order type diffraction grating DG rises, the diffraction pattern PT as a whole will be close to the shape of a right triangle, or the shape of a brazed hologram. This increases the diffraction efficiency.

On the other hand, increasing the integer “p” or “q” in the above equations (4)-(5) or (6)-(8) lowers the efficiency due to diffraction around vertical surfaces of the protruding portions of the diffraction pattern PT, slight bumps on the vertical surfaces of the protruding portions and the like.

Accordingly, it is desirable that the integers “p” and “q” satisfy the following conditions:

By the way, if the phase difference φ1 for one step of the protruding portion (the equations (4)-(5)) becomes different from “2π×a whole number” as indicated as “φ1=(2π×(p+ε))” (wherein ε is less than one, indicating a difference), or if the phase difference φs for “s” steps of the protruding portion (the equations (6)-(7)) becomes different from “2π×a whole number” as indicated as “φs=(2π×(q+ε))”, that lowers the transmission or diffraction efficiency.

The amount of lowering of the efficiency T (or the degree of efficiency reduction) is calculated as follows:
T(ε)≈T0 cos(ε(s−1)2π)2(10)
where T(ε) represents the amount of lowering of the efficiency, and T(0) represents the efficiency when the difference ε does not occur. In order to keep the amount of lowering T(ε) below 20%, the following condition may need to be satisfied:

Satisfying the equation (9) about the integers “p” and “q” and the equation (11) about the difference ε contributes to the increase of the transmission and diffraction efficiencies.

(2-2) Optical Path Difference for a Plurality of Wavelengths

The optical path difference for a plurality of wavelengths λ on the diffraction grating DG will be described: the wavelengths include the CD-type wavelength λc, the DVD-type wavelength λd and the BD-type wavelength λb.

The following describes whether the wavelengths λ should be diffracted at the diffraction grating DG i.e., whether the zero-order type or the first-order type.

On the basis that the diffraction gratings DG can be formed on the upper layer section20A and the lower layer section20B of the diffraction element20(FIG. 5A), assume that both an upper layer diffraction grating DGx and a lower layer diffraction grating DGy may be formed. Also this is designed on the basis that the objective lens21is designed to be suitable for the BD-type optical beam Lb, and the CD-type optical beam Lc is stronger than the BD-type optical beam Lb.

In this case, as shown inFIGS. 7A and 7B, there are two types of combination: a first combination (FIG. 7A) in which the both diffraction gratings DGx and DGy do not diffract the BD-type wavelength λb while the DVD- and CD-type wavelengths λd and λc are diffracted by the lower layer and upper layer diffraction gratings DGy and DGx, respectively; and a second combination (FIG. 7B) in which the both diffraction gratings DGx and DGy do not diffract the BD-type wavelength λb, and the DVD-type wavelength λd is diffracted by the lower layer diffraction grating DGy while the CD-type wavelength λc is diffracted by both the upper layer and lower layer diffraction gratings DGx and DGy.

That means that there are two possible choices to the diffraction grating DG as shown inFIG. 7C: a first diffraction grating DG1that only diffracts the wavelength λc while the wavelengths λb and λd are not diffracted; and a second diffraction grating DG2that diffracts the wavelengths λd and λc while the wavelength λb is not diffracted.

Since the same calculation method can be used for both the diffraction grating that only diffracts the wavelength λd and the diffraction grating that only diffracts the wavelength λc, the first diffraction grating DG1will be described as one that only diffracts the wavelength λc for ease of explanation.

By the way, the materials M1and M2, from which the diffraction grating DG are made, are similar to general materials which the optical beam L passes through, as shown inFIG. 8. This means that their diffraction index n1and n2vary according to wavelengths λ, causing dispersion. In addition, the refraction index n1changes with wavelengths in a different manner from the refraction index n2. As shown inFIG. 9, the difference between these refraction indexes also varies according to wavelengths i.e., having dispersion.

Accordingly, the following values are different from each other: the refraction indexes difference for the BD-type wavelength λb or (n2(λb)−n1(λb)); the refraction indexes difference for the DVD-type wavelength λd or (n2(λd)−n1(λd)); and the refraction indexes difference for the CD-type wavelength λc or (n2(λc)−n1(λc)).

That means that, when designing the diffraction grating that supports a plurality of wavelengths λ, the fact that the refraction indexes vary according to wavelengths λ may need to be considered.

(2-2-1) First Diffraction Grating

The following describes the first diffraction grating DG1(FIG. 7C) that only diffracts the CD-type wavelength λc, not the BD- and DVD-type wavelengths λb and λd. The diffraction grating DG1satisfies the following condition to only diffract the CD-type wavelength λc:

This condition (12) is based on the above equations (4)-(7). Each side of the above equation (12) is then divided by 2π to obtain the ratio as follows:

Each term in the left side of the above equation (13) is then multiplied by λb/d to obtain the following equation (14):

In this case, as shown in an equation (15) below, the refraction indexes differences are represented as δb, δd and δc while the ratio of wavelengths are represented as ρbd and ρbc: They replace each term in the left side of the equation (14). Considering that each term in the right side of the equation (14) includes εb, εd or εc which indicates the difference from integers, the equation (14) can be transformed into the following equation (16).

Each term in the equation (16) includes the following except the height d of the steps: the wavelengths λ; the refraction indexes n of materials M for each wavelength λ; the number of steps s; the integers p and q; and the differences from integers ε. This means that the equation (16) is not associated with the height d.

In addition, replacing the integer q of the equation (8) with other integer qc presents a condition that “the integer qc, which is based on the diffraction-target wavelength λc, is prime to s or the number of steps” as follows:
gcd(s,qc)=1  (17)

When the equations (16), (17) and (11) are satisfied, the diffraction grating DG only diffracts the wavelength λc, not the wavelengths λb and λd. The materials M1and M2are therefore selected such that their refraction indexes satisfy the equations (16), (17) and (11).

(2-2-2) Second Diffraction Grating

The following describes the second diffraction grating DG2(FIG. 7C) that diffracts the DVD- and CD-type wavelength λd and λc, not the BD-type wavelengths λb. The diffraction grating DG2satisfies the following condition, which is slightly different from that of the first diffraction grating, to diffract the DVD- and CD-type wavelength λd and λc:

This condition (18) is based on the above equations (4)-(7). In a similar way to that of the first diffraction grating, each side of the above equation (18) is divided by 2π to obtain the ratio, and each term in the left side of the resultant equation is then multiplied by λb/d to obtain the following equation (19):

The terms of the equation (15) replace terms in the left side of the equation (19). Considering that each term in the right side of the equation (19) includes εb, εd or εc which indicates the difference from integers, the equation (19) can be transformed into the following equation (20):
δb:s·δd·ρbd:s·δc·ρbc=pb+εb:qd+εd:qc+εc(20)

Replacing the integer q of the equation (8) with other integer qd or qc presents a condition that “the integer qd, which is based on the diffraction-target wavelength λd, is prime to s or the number of steps” and a condition that “the integer qc, which is based on the diffraction-target wavelength λc, is prime to s or the number of steps” as follows:
gcd(s,qd)=1,
gcd(s,qc)=1  (21)

Each term in the equation (20) does not include the height d of the steps: the equation (20) is not associated with the height d.

When the equations (20), (21) and (11) are satisfied, the diffraction grating DG diffracts the wavelengths λd and λc, not the wavelengths λb. The materials M1and M2are therefore selected such that their refraction indexes satisfy the equations (20), (21) and (11).

(2-3) Examples of Designing Diffraction Gratings

An example of designing the diffraction grating DG (FIG. 6) will be described. In this case, the diffraction grating DG to be designed is similar to the above first diffraction grating that only diffracts the CD-type wavelength λc, not the BD- and DVD-type wavelengths λb and λd.

In this case, the design condition is for example defined as follows: the diffraction pattern has three steps for each protruding part, the BD-type wavelength λb is 408 nm, the DVD-type wavelength λd is 655 nm, and the CD-type wavelength λc is 785 nm.

In this manner, the number of steps s is set at a prime number or three. This is because it is easy to satisfy the equation (17).

Using the above design condition, the refraction indexes of materials are applied to the left side of the equation (16), and it is checked that whether or not the equation (17) is satisfied. In addition, the following equations are also checked: whether the equation (11) is satisfied or not with the differences from integers εb, εd and εc i.e., whether or not the ratio of the left side of the equation (16) can be substantially expressed by integers.

When the materials M1and M2are, respectively, polyolefin resin and acrylic ultraviolet curable resin that have the refraction indexes as shown inFIG. 10, the following equation (22) is obtained as a result of substituting these refraction indexes for the corresponding parts of the equation (16):
δb:δd·ρbd:s·δc·ρbc=4+0.159:2+0.008:5+(−0.095)  (22)

In addition, the following equation (23) is obtained as a result of substituting each value for the corresponding parts of the equation (17):
gcd(3,5)=1  (23)

It is evident from the equations (22) and (23) that both the equations (17) and (11) are satisfied. The attainment of the goal means the diffraction grating DG only diffracting the CD-type wavelength λc without diffraction of the BD- and DVD wavelengths λb and λd.

FIG. 11shows the result of calculating diffraction efficiencies on the diffraction grating DG in which the height of the step is represented as “d”: Based on the result, the “d” is set at 6.53 μm to increase the diffraction efficiencies for all the wavelengths λb, λd and λc.

In this manner, the design of the diffraction grating DG includes the process of: substituting the refraction indexes of materials for the corresponding parts of the left side of the equation (16); and finding a condition that satisfies the equation (17) and the equation (11) regarding the differences from integers εb, εd and εc. This allows designing the diffraction grating DG that only diffracts certain wavelengths.

By the way, the interval of the protruding portions of the diffraction pattern PT is determined by performing a predetermined calculation based on the distance from the diffraction element20to the objective lens21(FIG. 4), the target diffraction angles of the optical beam L and the like.

(3) Operation and Effect

To design the diffraction grating DG that only diffracts the CD-type wavelength λc without any diffraction of the BD- and DVD-type wavelengths λb and λd, the refraction indexes n of possible materials M1and M2for each wavelength λ are applied to the left side of the equation (16).

The desired diffraction grating DG is attained when the right side of the equation (16) is simply expressed by integers and the equation (17) is satisfied. In sum, the materials M1and M2are suitable for the diffraction grating DG.

Using the equations (16), (17) and (11), it is easily checked whether the materials M1and M2are suitable for the diffraction grating DG. This allows easy selection of materials M1and M2to build the target diffraction grating DG.

In addition, when the equation (11) is satisfied, the diffraction and transmission efficiencies for each wavelength λ of the optical beam are maintained.

Moreover, the height of step d is not specified in the equation (16). This allows selection of the materials M1and M2before planning the dimension of the diffraction pattern PT. After selecting the materials M1and M2the height of step d is properly determined based on the refraction indexes n1and n2of the materials M1and M2and the like.

Abbe number, a measure used for designing optical components may be applied to selection of materials M1and M2. However, the Abbe number are not suitable for the UV curable resin: While it is a measure of dispersion of glass materials and the like, the Abbe number does not work well for abnormal dispersion of the UV curable resin.

On the other hand, the equation (16) works well for abnormal dispersion of the UV curable resin because it uses the real refraction indexes n of materials M1and M2regarding diffracted and non-diffracted wavelengths λ. This allows designing the diffraction grating DG despite the abnormal dispersion.

The above configuration makes this possible: To design the diffraction grating DG that only diffracts a certain wavelength λ, the refraction indexes n of possible materials M1and M2for each wavelength λ are applied to the left side of the equation (16); and, if the equation (11) is satisfied to express the right side of the equation (16) in a simple integer ratio and the equation (17) is satisfied at the same time, the materials M1and M2are determined as suitable for the diffraction grating DG. This allows easy selection of the materials M1and M2for the desired diffraction grating DG.

(4) Other Embodiments

In the above-noted embodiments, the diffraction grating DG is designed to only diffract the CD-type wavelength λ without any diffraction of the BD- and DVD-type wavelengths λb and λd. However the present invention is not limited to this. The diffraction grating DG may be designed to diffract other wavelengths, such as diffracting only the DVD-type wavelength λd without diffracting the BD- and CD-type wavelengths λb and λc.

In the above-noted embodiments, the diffraction grating DG includes the materials M1and M2attached to one another. However the present invention is not limited to this. The diffraction grating DG may not include the material M2.

In this case, the air surrounding the material M1may be regarded as the material M2whose refraction index n is “1”: For example, when designing the DVD-type diffraction grating DGd on the lower layer section20B of the diffraction element20(FIG. 5A), the equation (16) is defined on the basis that only the wavelength λd is diffracted. In addition, the refraction index n2of the material M2is set at “1”.

Furthermore, in the above-noted embodiments, the BD-type wavelength of 408 nm, the DVD-type wavelength of 655 nm and the CD-type wavelength of 785 nm are applied. However the present invention is not limited to this. It may include other wavelengths or other formats.

Furthermore, in the above-noted embodiments, after selecting the wavelengths to be diffracted out of three types of wavelengths (BD-, DVD and CD-types), the materials M1and M2are selected based on the equations (16) and (20). However the present invention is not limited to this. For example, after selecting the wavelengths to be diffracted out of four types of wavelengths, the materials M1and M2may be selected based on the equations (12)-(16) or the transformed equations (17)-(20).

Furthermore, in the above-noted embodiments, the amount of lowering of the efficiency T, or T(ε), is maintained below about 20%. However the present invention is not limited to this. The amount of lowering T(ε) may be maintained below 10 or 15%. In addition, instead of the equation (11), ε is defined in other ways such as ε<0.1.

Furthermore, in the above-noted embodiments, the CD-type diffraction grating DGc of the upper layer section20A of the diffraction element20(FIG. 5A) only diffracts the CD-type wavelength λc while the DVD-type diffraction grating DGd of the lower layer section20B only diffracts the DVD-type wavelength λd. However the present invention is not limited to this. For example, the diffraction grating DG of the lower layer section20B may only diffract the CD-type wavelength λc while the diffraction grating DG of the upper layer section20A may diffract the DVD- and CD-type wavelengths λd and λc. Other combinations may be applied.

Furthermore, in the above-noted embodiments, the diffraction element20incorporated in the objective lens unit9of the optical pickup7of the optical disc device1is designed by the above method. However the present invention is not limited to this. The diffraction element20, the objective lens unit9or the optical pickup7may be incorporated in other devices.

The diffraction element, objective lens unit, optical pickup, optical disc apparatus and design method for diffraction elements can be applied to other optical devices that support a plurality of wavelengths of optical beams.