Marking method and marking apparatus using multiple photon absorption, marked optical element manufactured by using the marking method and the marking apparatus

A marking apparatus comprising:

• a stage for mounting a marking object;
• a light source for producing a laser beam having a wavelength range so chosen that the laser beam can transmit through a material forming the marking object;
• an optical system for converging the laser beam emitted from the light source into the marking object, then effecting a multiple photon absorption;
• and
• a moving mechanism for moving the converging position in a manner such that characteristic-changed portions formed by virtue of the multiple photon absorption, will form, in the converging positions of the laser beam, a diffraction grating capable of diffracting a visible light.

A marking method comprising:

• a first step of irradiating a marking object by a pulse laser beam while at the same time changing NA of an objective lens and an energy per pulse, thereby forming a characteristic-changed area by virtue of move of a light converging spot;
• a second step of obtaining a relationship among the length of the characteristic-changed area, an NA and an energy per pulse;
• a third step of determining the length of a characteristic-changed area to be formed;
• a fourth step of determining an NA and an energy per pulse, in accordance with the relationship obtained in the second step, also in accordance with the length of a characteristic-changed area to be formed;
• and
• a fifth step of converging a laser beam on the marking object to form a characteristic-changed area therein, by using the determined NA and the determined energy per pulse.