Method for designing an optical pickup

A method for determining a specification of an error of an emission direction in each emitting portion of a monolithic laser diode used in an optical pickup, the optical pickup including:

• the monolithic laser diode that emits
• a first light beam for recording to a disc and
• a second light beam for playing back from a disc or for recording to a disc;
• a substrate;
• a first photodetective portion provided on the substrate for receiving the first light beam;
• a second photodetective portion provided on the substrate for receiving the second light beam;
• and
• an objective lens,
• the method comprising:
• a step of determining light beam diameters immediately after the first and second light beams exit from the objective lens;
• a step of determining beam spot diameters in the first and second photodetective portions;
• a step of determining, with respect to the first light beam, a variation in sensitivity, to emission angle, of a deviation of a gravity center of intensity of light exiting from the objective lens;
• a step of determining, with respect to the second light beam, sensitivity or a variation therein, to emission angle, of a deviation of a gravity center of intensity of light exiting from the objective lens;
• a step of determining, with respect to each of the first and second light beams, sensitivity of a deviation of light reception balance to a deviation of the photodetective portion;
• and
• a step of determining the specification of the error of the emission direction in each emitting portion of the monolithic laser diode such that at least one of formulae (A) and (B) is fulfilled, based on the light beam diameters immediately after exiting from the objective lens, the beam spot diameters in the photodective portions, the variation in sensitivity, to emission angle, of the deviation of the gravity center of the intensity of light exiting from the objective lens with respect to the first light beam, the sensitivity or variation therein, to emission angle, of the deviation of the gravity center of the intensity of light exiting from the objective lens with respect to the second light beam, and the sensitivity of the deviation of the light reception balance to the deviation of the photodetective portion thus determined;
• |(Δθ⊥1×Δθ1×Dpd1/Dob1−Δθ⊥2×Δθ2×Dpd2/Dob2)×Kpd2|≦α (A)
• |(Δθ⊥1×Δθ1×Dpd1/Dob1−Δθ⊥2×Δθ2×Dpd2/Dob2)×Kpd1|≦α (B)
• where, let variables corresponding to the first light beam be followed by a subscript “1” and variables corresponding to the second light beam be followed by a subscript “2”,
• Δθ⊥ represents an emission angle;
• Δθ represents sensitivity, to the emission angle, of a deviation of a gravity center of intensity of light exiting from the objective lens;
• Dob represents a diameter of the light beam immediately after exiting from the objective lens;
• Dpd represents a diameter of a spot formed by the light beam on the photodetective portion;
• Kpd represents sensitivity of a deviation of light reception balance to a deviation of the photodetective portion;
• and
• α represents a tolerable value for a remnant deviation of light reception balance.