Low bias drift modulator with buffer layer
View Patent ↗The invention relates to an electro-optic modulator structure containing an additional set of bias electrodes buried within the device for applying bias to set the operating point. Thus the RF electrodes used to modulate incoming optical signals can be operated with zero DC bias, reducing electrode corrosion by galvanic and other effects that can be present in non-hermetic packages. The buried bias electrodes are also advantageous in controlling charge build-up with consequent improvement in drift characteristics. The bias electrode material is useful for routing bias signals inside the device, in particular to external terminals, as well as forming encapsulating layers to permit operation in non-hermetic environments, thereby lowering manufacturing costs. Embodiments using both X-cut and Z-cut lithium niobate (LiNbO 3 ) are presented. For the latter, the bias electrodes can be split along their axis to avoid optical losses.
1. An electro-optical device comprising, a substrate; an optical waveguide embedded within said substrate, said optical waveguide further comprising a first optical pathway and a second optical pathway; a bias electrode layer formed on said substrate; a buffer layer formed on at least a portion of said bias electrode layer and said substrate; and, an RF electrode layer formed on said buffer layer, wherein the RF electrode is DC isolated from the bias electrode.
2. The electro-optic device of claim 1 wherein the bias electrode layer forms bias electrodes and wherein the RF electrode layer forms RF electrodes that are in substantial alignment with the RF electrodes.
3. The electro-optic device of claim 1 wherein the bias electrode layer has an electrical resistivity substantially higher than that of the RF electrode layer, but substantially lower than the substrate and the buffer layer.
4. The electro-optic device of claim 2 wherein an inter-electrode gap of the bias electrodes is less than an inter-electrode gap of the high conductivity RF electrodes.
5. The electro-optic device of claim 1 further comprising at least one conducting via through the buffer layer for providing a bias signal to the bias layer.
6. The electro-optic device of claim 2 further comprising a plurality of conducting vias through the buffer layer for reducing the series resistance of the bias electrodes.
7. The electro-optic device of claim 1 wherein the bias signal layer and the RF signal layer are connected to form a common connection.
8. The electro-optic device of claim 2 wherein the bias electrodes are formed by the ion implantation of a dopant material which increases the conductivity of the host material.
9. The electro-optic device of claim 8 wherein the bias electrodes formed by the ion implantation have a thickness equal to that of the buffer layer, thereby forming a conductive path to the device surface.
10. The electro-optical device of claim 1 , wherein said bias electrode layer is comprised of a semi-conductor.
11. The electro-optical device of claim 10 , wherein said semi-conductor conducts electricity at low-frequencies and is a dielectric at high-frequencies.
12. The electro-optic device of claim 3 wherein the RF electrode layer comprises gold.
13. The electro-optic device of claim 3 wherein the electrical resistivity of the bias electrode layer is between about 10 4 and 10 6 Ohm-cm.
14. The electro-optic device of claim 2 wherein the bias electrodes are formed by ion implantation of a dopant in one of the substrate and the buffer layer to increase a local conductivity thereof.