Method for fabricating nonlinear optical waveguide with gradient refractive index distribution

A method for fabricating an optical waveguide with a focusing capability, comprising:

• (S1) preparing a desired nonlinear optical crystal, and determining a type of first heavy ions, a type of second heavy ions and parameters related to an irradiation process;
• (S2) accelerating the first heavy ions to generate a first ion beam;
• and bombarding the desired nonlinear optical crystal with the first ion beam to form a first refractive index dip at a first depth along a depth direction of the nonlinear optical crystal, so as to obtain a primary processed crystal;
• (S3) accelerating the second heavy ions to generate a second ion beam;
• and bombarding the primary processed crystal with the second ion beam to form a second refractive index dip at a second depth along a depth direction of the primary processed crystal, so as to obtain a secondary processed crystal;
• and
• (S4) segmenting the secondary processed crystal to obtain the optical waveguide;
• wherein the first heavy ions and the second heavy ions are both chloride ions;
• the desired nonlinear optical crystal is a KTiOPO4 (KTP) crystal;
• step (S4) comprises:
• (S4.1) ablating, by a surface ablation device, an upper surface of the secondary processed crystal to form a first optical waveguide side;
• and
• (S4.2) ablating, by the surface ablation device, the upper surface of the secondary processed crystal to form a second optical waveguide side, wherein a portion enclosed by the first optical waveguide side and the second optical waveguide side is the optical waveguide;
• wherein each of the first optical waveguide side and the second optical waveguide side extends from a light-input end surface of the secondary processed crystal to a light-output end surface of the secondary processed crystal;
• along a direction from the light-input end surface to the light-output end surface, the first optical waveguide side comprises a first input section, a first transition section and a first output section in sequence, and the second optical waveguide side comprises a second input section, a second transition section and a second output section in sequence;
• along the direction from the light-input end surface to the light-output end surface, the first input section has the same length as the second input section, the first input section is parallel to the second input section, the first transition section has the same length as the second transition section, a spacing between the first transition section and the second transition section is decreasing, the first output section has the same length as the second output section, and the first output section is parallel to the second output section;
• and
• a titanium sapphire femtosecond laser processing system serves as the surface ablation device to ablate the secondary processed crystal.