Integrated optical mode shape transformer and method of fabrication

• US 7,359,593 B2
• Filed: 05/31/2006
• Issued: 04/15/2008
• Est. Priority Date: 10/09/2003
• Status: Expired due to Term

• Current Assignee: Infinera Corp.
• Original Assignee: Infinera Corp.
• Inventors: Little, Brent E.
• Primary Examiner(s): Wong; Tina M.
• Application Number:
  US11/443,561
• Publication Number:
  US 20060285797A1
• Time in Patent Office: 685 Days
• Field of Search: None
• US Class Current: 385/30
• CPC Class Codes: G02B 6/1228 Tapered waveguides, e.g. integrated spot-size transformers for coupling with fibres G02B6/305
• CPC Class Codes: G02B 6/305 and having an integrated mode-size expanding section, e.g. tapered waveguide

Abstract

An integrated optical mode transformer provides a low loss interconnection between an optical fiber and an integrated optic waveguide having a spot size different from that of the fiber. The mode transformer is comprised of two waveguide layers, an upper layer and a lower layer, with the upper layer being contiguous to the lower layer. The lower layer is the integrated optic waveguide layer forming the optical circuit. The input dimensions of the composite two-waveguide structure supports a fundamental mode that accepts all of the light present on the optical fiber. The upper waveguide layer is tapered down from an input width to an output width and then terminates in such a way that at the termination substantially all of the input optical power resides in the lower waveguide layer. The two-waveguide layer structure is fabricated by deposition and planarization techniques.

Claims

1. An optical chip connectable to a waveguide having a spot size, the optical chip comprising:

a substrate;

an optical circuit on the substrate; and

an optical mode transformer on the substrate and having an input for coupling to the waveguide, and an output coupled to the optical circuit, the optical mode transformer, comprising;

a first waveguide layer having a first thickness and being constructed of a high contrast waveguide, the first waveguide layer coupled to the optical circuit; and

a second waveguide layer, at least a portion of the second waveguide layer being contiguous with at least a portion of the first waveguide layer, the first and second waveguide layers together forming a composite waveguide having a mode matching section adjacent to the input of the optical mode transformer, the mode matching section having a spot size matching the spot size of the waveguide to capture light from the waveguide, and an adiabatic section in which the second waveguide layer is tapered, with a larger end of the second waveguide layer receiving light from the mode matching section and a mode size of propagating light in the second waveguide layer being reduced along the taper of the second waveguide layer to couple the propagating light into the first waveguide layer and to the optical circuit on the substrate.

2. The optical chip of claim 1, wherein the composite waveguide further defines a lateral mode reduction section between the mode matching section and the adiabatic section and wherein both the first and second waveguide layers are simultaneously reduced in the lateral mode reduction section.

3. The optical chip of claim 1 wherein the first and second waveguide layers are constructed of materials having a similar refractive index.

4. The optical chip of claim 1 wherein the first and second waveguide layers are constructed of materials having different refractive indexes.

5. A method of fabricating an optical chip, comprising the steps of:

depositing a first waveguide and an optical circuit on a substrate such that the first waveguide is connected to the optical circuit, the first waveguide being constructed of a high contrast waveguide and having a first thickness and a first refractive index; and

fabricating a second waveguide having a second thickness and a second refractive index on the first waveguide where at least a portion of the second waveguide is contiguous with at least a portion of the first waveguide and the second waveguide having a width that is tapered inwardly toward the optical circuit, a portion of the first and second waveguides forming a mode matching section positioned to receive tight from an external waveguide positioned externally of the optical chip, the mode matching section having a cross-sectional size matching a cross-sectional size of the external waveguide.

6. The method of claim 5 comprising the further step of forming the first and second waveguides with materials having similar refractive indexes.

7. The method of claim 5 comprising the further step of forming the first and second waveguides with materials having different refractive indexes.

8. The method of claim 5 wherein the first and second waveguides having a width in the mode matching section corresponding to that of the external waveguide.

9. The method of claim 8 wherein the sum of the first thickness of the first waveguide and the second thickness of the second waveguide are similar to the fiber spot size.

10. A method of coupling an input optical signal from a fiber into an integrated optical high index contrast first waveguide of an optical chip having a spot size different from that of the fiber, comprising the steps of:

aligning the fiber with a mode matching section of an integrated optical mode transformer of the optical chip, the mode matching section having a spot size matching the snot size of the fiber, the integrated optical mode transformer also including a second waveguide contiguous with the first waveguide to form a composite waveguide, the composite waveguide having an adiabatic section in which the second waveguide tapers inwardly relative to the first waveguide to reduce the mode size of propagating light in the second waveguide layer and to cause optical power of the input optical signal to transfer from the second waveguide to the first waveguide; and

securing the optical mode transformer in the aligned position.

11. The method of claim 10 wherein the optical mode transformer is defined further wherein the composite waveguide formed from the first and second waveguides defines a lateral mode reduction section between the mode matching section and the adiabatic section and wherein both the first and second waveguides are simultaneously reduced in the lateral mode reduction section.

12. The method of claim 10 wherein in the mode matching section the first and second waveguides each have a width corresponding to that of a predetermined fiber spot size.

13. The method of claim 10 wherein the first and second waveguides are constructed of materials having a similar refractive index.

14. The method of claim 10 wherein the first and second waveguides are constructed of materials having different refractive indexes.

15. The method of claim 10 wherein a width of the second waveguide tapers to a terminal end along the adiabatic section.

16. The method of claim 15 wherein a width of the first waveguide is not tapered along the adiabatic section.

17. The method of claim 10 wherein the first and second waveguides have respective lengths where the length of the second waveguide is less than the length of the first waveguide forcing the propagating light to evanescently couple into the first waveguide.