DIFFRACTION ELEMENT, OBJECTIVE LENS UNIT, OPTICAL PICKUP AND OPTICAL DISC APPARATUS

Abstract

A diffraction element for diffracting a particular wavelength of incident optical beam includes: a base section made from a first resin and provided with a predetermined diffraction pattern; and a cover section made from a second resin and covering the diffraction pattern, wherein a rate of change of refraction index of the first resin is substantially the same as a rate of change of refraction index of the second resin in a temperature range between a first temperature and a second temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram illustrating the overall configuration of an optical disc device;

FIG. 2 is a schematic diagram illustrating the configuration of an optical pickup;

FIG. 3 is a schematic diagram illustrating the configuration of an objective lens unit;

FIG. 4 is a schematic diagram illustrating light paths inside the objective lens unit;

FIGS. 5A and 5B are schematic diagrams illustrating the configuration of a diffraction element;

FIG. 6 is a schematic diagram illustrating a difference between optical paths on a diffraction grating;

FIGS. 7A and 7B are schematic diagrams illustrating refraction indexes for each wavelength;

FIG. 8 is a schematic diagram illustrating dependency of refraction indexes to temperatures;

FIG. 9 is a table showing refraction indexes differences with temperatures;

FIG. 10 is a table illustrating changes in diffraction efficiencies in different temperatures; and

FIG. 11 is a table illustrating the variation of diffraction efficiencies.

Claims

1. A diffraction element for diffracting a particular wavelength of incident optical beam, the diffraction element including: a base section made from a first resin and provided with a predetermined diffraction pattern; anda cover section made from a second resin and covering the diffraction pattern, whereina rate of change of refraction index of the first resin is substantially the same as a rate of change of refraction index of the second resin in a temperature range between a first temperature and a second temperature.

2. The diffraction element according to claim 1, wherein the first temperature is −40 degrees Celsius while the second temperature is 75 degrees Celsius.

3. The diffraction element according to claim 1, wherein a difference between the rate of change of refraction index of the first resin and the rate of change of refraction index of the second resin is less than ±0.0005 in the temperature range of −40 degrees Celsius to 75 degrees Celsius.

4. The diffraction element according to claim 1, wherein when the optical beam having a plurality of wavelengths is received, the rate of change of refraction index of the first resin is substantially the same as the rate of change of refraction index of the second resin in a temperature range between the first temperature and the second temperature.

5. The diffraction element according to claim 1, wherein: a first wavelength of the optical beam is diffracted; anda refraction indexes difference for the first wavelength of the optical beam is different from that of a second wavelength of the optical beam, the refraction indexes difference being between the refraction index of the first resin and the refraction index of the second resin.

6. The diffraction element according to claim 5, wherein the first wavelength is substantially equal to the second wavelength multiplied by a whole number.

7. The diffraction element according to claim 1, wherein glass transition points of the first resin and the second resin are greater or equal to 85 degrees Celsius.

8. The diffraction element according to claim 1, wherein the first resin and the second resin are different materials selected from thermoplastic resin, thermosetting resin and ultraviolet curable resin.

9. The diffraction element according to claim 7, wherein the first resin is ultraviolet curable resin while the second resin is thermoplastic resin.

10. An objective lens unit including: a diffraction element for diffracting a particular wavelength of incident optical beam from a light source, the diffraction element including a base section made from a first resin and provided with a predetermined diffraction pattern and a cover section made from a second resin and covering the diffraction pattern; andan objective lens integrated with the diffraction element and emitting the optical beam to an optical disc after collecting the optical beam from the diffraction element, whereina rate of change of refraction index of the first resin is substantially the same as a rate of change of refraction index of the second resin in a temperature range between a first temperature and a second temperature.

11. An optical pickup including: a light source emitting an optical beam;an objective lens emitting the optical beam to an optical disc after collecting the optical beam; anda diffraction element for diffracting a particular wavelength of the incident optical beam to the objective lens, the diffraction element including a base section made from a first resin and provided with a predetermined diffraction pattern and a cover section made from a second resin and covering the diffraction pattern, whereina rate of change of refraction index of the first resin is substantially the same as a rate of change of refraction index of the second resin in a temperature range between a first temperature and a second temperature.

12. An optical disc apparatus including: a light source emitting an optical beam;an objective lens emitting the optical beam to an optical disc after collecting the optical beam;a diffraction element for diffracting a particular wavelength of the incident optical beam to the objective lens, the diffraction element including a base section made from a first resin and provided with a predetermined diffraction pattern and a cover section made from a second resin and covering the diffraction pattern; anda drive section to have the objective lens focused on a target track of the optical disc, whereina rate of change of refraction index of the first resin is substantially the same as a rate of change of refraction index of the second resin in a temperature range between a first temperature and a second temperature.