Multimode long period fiber bragg grating machined by ultrafast laser direct writing

A fiber Bragg grating for a predetermined wavelength band, comprising:

• an optical fiber core having a core index of refraction;
• a substantially cylindrical surface;
• a longitudinal core axis;
• and
• a core radius;
• and
• a cladding layer formed of a non-photosensitive material on the substantially cylindrical surface of the optical fiber core, the non-photosensitive material of the cladding layer having an initial cladding index of refraction lower than the core index of refraction of the optical fiber core, the cladding layer includes;
• an outer cladding radius;
• and
• a plurality of index-altered portions having an altered index of refraction different from the initial cladding index of refraction;
• wherein;
• each of the plurality of index-altered portions of the cladding layer extends into the cladding layer from the substantially cylindrical surface of the optical fiber core;
• the index-altered portion outer radii of the plurality of index-altered portions vary between different index-altered portions in the longitudinal direction of the optical fiber core;
• and
• the plurality of index-altered portions are arranged within the non-photosensitive material of the cladding layer to form an apodized Bragg grating structure.

A wavelength stabilized, high power, uncooled laser source, comprising:

• at least one high power laser;
• and
• a multimode optical fiber optically coupled to the at least one high power laser, the multimode optical fiber including a multimode core formed of a non-photosensitive material having an initial index of refraction, the multimode core including;
• a substantially cylindrical surface;
• a longitudinal core axis;
• a core radius;
• and
• a plurality of index-altered portions having an altered index of refraction different from the initial index of refraction;
• wherein the plurality of index-altered portions are arranged within the non-photosensitive material of the multimode core to form a long period Bragg grating structure for reflecting a predetermined fraction of light in a predetermined wavelength band propagating in the multimode core back into the at least one high power laser to lock an output wavelength band of the wavelength stabilized, high power, uncooled laser source to the predetermined wavelength band.

A multimode optical fiber with a helical fiber Bragg grating, comprising:

• a multimode optical fiber core formed of a non-photosensitive material having an initial index of refraction, the multimode optical fiber core includes;
• a substantially cylindrical surface;
• a longitudinal core axis;
• a core radius;
• and
• a helical index-altered portion having an altered index of refraction different from the initial index of refraction;
• wherein;
• the helical index-altered portion includes a longitudinal index-altered portion axis coaxial to the longitudinal core axis of the multimode core, an index-altered portion outer radius, an index-altered portion inner radius which is less than the index-altered portion outer radius, and a longitudinal pitch;
• and
• the helical index-altered portion is arranged within the non-photosensitive material of the multimode optical fiber core to form a long period Bragg grating structure.

A fiber Bragg grating for a predetermined wavelength band, comprising:

• an optical fiber core having a core index of refraction;
• a substantially cylindrical surface;
• a longitudinal core axis;
• and
• a core radius;
• and
• a cladding layer formed of a non-photosensitive material on the substantially cylindrical surface of the optical fiber core, the non-photosensitive material of the cladding layer having an initial cladding index of refraction lower than the core index of refraction of the optical fiber core, the cladding layer includes;
• an outer cladding radius;
• and
• a plurality of index-altered portions having an altered index of refraction different from the initial cladding index of refraction;
• wherein;
• each of the plurality of index-altered portions of the cladding layer extends into the cladding layer from the substantially cylindrical surface of the optical fiber core;
• each of the plurality of index-altered portions is an annular arcuate segment including:
• an index-altered portion inner radius which is equal to the core radius of the optical fiber core;
• an index-altered portion outer radius which is less than or equal to the outer cladding radius of the cladding layer;
• and
• an angular extent less than 180° around the longitudinal core axis of the optical fiber core;
• at least one of the index-altered portion outer radii or the angular extent of the plurality of index-altered portions varies between different index-altered portions in the longitudinal direction of the optical fiber core;
• and
• the plurality of index-altered portions are arranged within the non-photosensitive material of the cladding layer to form an apodized Bragg grating structure.