A process for writing gratings on an optical fiber (14, 1516), the process comprising: surrounding the optical fiber (14, 1516) with an index-matching material (20a, 20b, 20c, 124, 1560), the index-matching material (20a, 20b, 20c, 124, 1560) having an index of refraction η surround, and the optical fiber (14, 1516) comprising: an outer radius R fiber; a fiber index of refraction η fiber; an offset core (18, 19) following a helical trajectory along an axial direction along the optical fiber (14, 1516), the offset core (18, 19) being offset from a center of the optical fiber (14, 1516) by R offset which is more than 40% of the radius of the optical fiber; and writing gratings on the offset core (18, 19) 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 (14, 1516) with an index-matching material (20a, 20b, 20c, 124, 1560), the index-matching material (20a, 20b, 20c, 124, 1560) having an index of refraction η surround, and the optical fiber (14, 1516) comprising: an outer radius R fiber; a fiber index of refraction η fiber; an offset core (18, 19) following a helical trajectory along an axial direction along the optical fiber (14, 1516), the offset core (18, 19) being offset from a center of the optical fiber (14, 1516) 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 (18, 19) following a helical trajectory along an axial direction along the optical fiber (14, 1516), the offset core (18, 19) being offset from a center of the optical fiber (14, 1516) by R offset which is more than 40% of the radius of the optical fiber; and writing gratings on the offset core (18, 19) 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.