Low loss fiber-to-chip interfaces for lithium niobate photonic integrated circuits

A method, comprising:

• using an optical fiber to direct a guided mode to an optical interface of a waveguide, the waveguide comprising an electro-optical material,
• wherein:
• the waveguide includes an elevated ridge and a slab underlying the elevated ridge, the elevated ridge and the slab extending along a central axis toward the optical interface;
• the elevated ridge and the slab each has a plurality of cross-sections along the central axis, each cross-section having a width measured perpendicular to the central axis, wherein the width of elevated ridge is smaller than the width of the slab for every cross-section along the central axis, wherein the elevated ridge is trapezoidal in cross-section,
• the elevated ridge includes a tapered portion having a first taper, wherein the cross-section of the elevated portion decreases along the central axis toward the optical interface;
• the slab includes a tapered portion having a second taper, wherein the cross-section of the slab decreases along the central axis toward the optical interface, wherein:
• the slab extends beyond the elevated ridge along the central axis to the optical interface,
• wherein an optical coupling loss associated with the optical coupling between the optical fiber and the waveguide is equal to or less than 1.7 dB/facet at an operating wavelength of 1570 nm, 1680 nm, or any wavelength therebetween.

An optical circuit, comprising:

• a waveguide comprising an electro-optical material,
• the waveguide including an elevated ridge and a slab underlying the elevated ridge, the elevated ridge and the slab extending along a central axis toward an optical interface;
• the elevated ridge and the slab each having a plurality of cross-sections along the central axis, each cross-section having a width measured perpendicular to the central axis, wherein the width of elevated ridge is smaller than the width of the slab for every cross-section along the central axis, wherein the elevated ridge is trapezoidal in cross-section,
• the elevated ridge including a tapered portion having a first taper, wherein the cross-section of the elevated portion decreases along the central axis toward the optical interface;
• the slab including a tapered portion having a second taper, wherein the cross-section of the slab decreases along the central axis toward the optical interface, wherein:
• the slab extends beyond the elevated ridge along the central axis to the optical interface, and
• wherein the first taper and the second taper, each independently, is defined by an exponential curve.