A process for writing gratings on an optical fiber , the process comprising: surrounding the optical fiber with an index-matching material , the index-matching material having an index of refraction η surround , and the optical fiber comprising: an outer radius R fiber ; a fiber index of refraction η fiber ; an offset core following a helical trajectory along an axial direction along the optical fiber , the offset core being offset from a center of the optical fiber by R offset which is more than 40% of the radius of the optical fiber; and writing gratings on the offset core longer than twenty centimeters (cm) with a grating strength variation that is less than 3 decibels (dB) through one twist period, the gratings having a maximum grating strength, the gratings having a minimum gratings strength, a ratio of the minimum grating strength to the maximum grating strength being: 1 − ρ η − 1 1 + ρ η − 1 , where η = n fiber / n surround , ρ < 1/ η , and ρ = R offset / R fiber , wherein the fiber index of refraction is one of: a core index of refraction; a cladding index of refraction; a coating index of refraction; a combination of a core index of refraction and a cladding index of refraction; a combination of a cladding index of refraction and a coating index of refraction; and a combination of a core index of refraction, a cladding index of refraction, and a coating index of refraction. surrounding the optical fiber with an index-matching material , the index-matching material having an index of refraction η surround , and the optical fiber comprising: an outer radius R fiber ; a fiber index of refraction η fiber ; an offset core following a helical trajectory along an axial direction along the optical fiber , the offset core being offset from a center of the optical fiber by R offset which is more than 40% of the radius of the optical fiber; and an outer radius R fiber ; a fiber index of refraction η fiber ; an offset core following a helical trajectory along an axial direction along the optical fiber , the offset core being offset from a center of the optical fiber by R offset which is more than 40% of the radius of the optical fiber; and writing gratings on the offset core longer than twenty centimeters (cm) with a grating strength variation that is less than 3 decibels (dB) through one twist period, the gratings having a maximum grating strength, the gratings having a minimum gratings strength, a ratio of the minimum grating strength to the maximum grating strength being: 1 − ρ η − 1 1 + ρ η − 1 , where η = n fiber / n surround , ρ < 1/ η , and ρ = R offset / R fiber , wherein the fiber index of refraction is one of: a core index of refraction; a cladding index of refraction; a coating index of refraction; a combination of a core index of refraction and a cladding index of refraction; a combination of a cladding index of refraction and a coating index of refraction; and a combination of a core index of refraction, a cladding index of refraction, and a coating index of refraction. a core index of refraction; a cladding index of refraction; a coating index of refraction; a combination of a core index of refraction and a cladding index of refraction; a combination of a cladding index of refraction and a coating index of refraction; and a combination of a core index of refraction, a cladding index of refraction, and a coating index of refraction. <EOS>