Diffraction element, optical head device including the diffraction element, optical information apparatus including the optical head device, system including the optical information apparatus and method of designing diffraction grating of the diffraction element

A method comprising designing a diffraction grating formed on only one face of a light-transmittable member in a diffraction element such that a first laser beam having a first wavelength λ1 and a second laser beam having a second wavelength λ2 are transmitted through the diffraction grating at first and second diffraction efficiencies, respectively;

• wherein the diffraction grating has a plurality of land portions each having a width Wa and a plurality of groove portions each having a width Wb and a depth h alternately arranged therein and has a duty ratio defined as {Wa/(Wa+Wb)} such that the first and second diffraction efficiencies are adjusted by using the duty ratio and the depth h of the diffraction grating as parameters;
• wherein the depth h of the diffraction grating is approximately expressed by the following equation (7):
• h=[Nλ1λ2/{λ1(n2−1)+λ2(n1−1)}] (7)
• in which “N” is a natural number, “n1” is a refractive index of the diffraction element for the first laser beam and “n2” is a refractive index of the diffraction element for the second laser beam;
• and
• wherein a zero-order diffraction efficiency of the first laser beam and a zero-order diffraction efficiency of the second laser beam are approximately equal to each other and a first-order diffraction efficiency of the first laser beam and a first-order diffraction efficiency of the second laser beam are approximately equal to each other.

An optical head device comprising:

• a first laser beam source for emitting a blue laser beam having a first wavelength λ1;
• a second laser beam source for emitting a red laser beam having a second wavelength λ2;
• an optical lens for condensing the blue laser beam or the red laser beam at a minute spot on an information recording face of an optical information medium;
• a photodetector for outputting, in response to the blue laser beam or the red laser beam reflected on the information recording face of the optical information medium, an electric signal corresponding to a quantity of the blue laser beam or the red laser beam;
• and
• a diffraction element including a light-transmittable member and a diffraction grating formed on at least one face of the light-transmittable member;
• wherein when the blue laser beam and the red laser beam are transmitted through the diffraction element, the diffraction element turns the blue laser beam and the red laser beam into first and second diffracted beams having first and second diffraction efficiencies, respectively;
• wherein the diffraction element functions to equalize the first and second diffraction efficiencies by only the one face of the diffraction element;
• wherein the diffraction grating has a first phase modulation amount ø1 for the blue laser beam and a second phase modulation amount ø2 for the red laser beam and the first and second phase modulation amounts ø1 and ø2 are, respectively, approximately expressed by the following equations (11) and (12):
• ø1=2πN1±Δø (11)
• ø2=2πN2±Δø (12)
• in which “N1” and “N2” are natural numbers and “Δø” is a phase variation amount;
• and
• wherein the photodetector receives the first or second diffracted beam so as to detect a servo signal.

An optical information apparatus comprising:

• an optical head device which includes a first laser beam source for emitting a blue laser beam having a first wavelength λ1, a second laser beam source for emitting a red laser beam having a second wavelength λ2, an optical lens for condensing the blue laser beam or the red laser beam at a minute spot on an information recording face of an optical information medium, a photodetector for outputting, in response to the blue laser beam or the red laser beam reflected on the information recording face of the optical information medium, an electric signal corresponding to a quantity of the blue laser beam or the red laser beam, and a diffraction element having a light-transmittable member and a diffraction grating formed on at least one face of the light-transmittable member;
• wherein when the blue laser beam and the red laser beam are transmitted through the diffraction element, the diffraction element turns the blue laser beam and the red laser beam into first and second diffracted beams having first and second diffraction efficiencies, respectively;
• wherein the diffraction element functions to equalize the first and second diffraction efficiencies by only the one face of the diffraction element;
• wherein the diffraction grating has a first phase modulation amount ø1 for the blue laser beam and a second phase modulation amount ø2 for the red laser beam and the first and second phase modulation amounts ø1 and ø2 are, respectively, approximately expressed by the following equations (11) and (12):
• ø1=2πN1±Δø (11)
• ø2=2πN2±Δø (12)
• in which “N1” and “N2” are natural numbers and “Δø” is a phase variation amount;
• wherein the photodetector receives the first or second diffracted beam so as to detect a servo signal;
• a motor for rotating the optical information medium;
• and
• an electric circuit for controlling and driving, on the basis of a signal received from the optical head device, the motor or one of the optical lens, the first laser beam source and the second laser beam source of the optical head device.