# US 10,120,135 B2 · Inverse taper waveguides for low-loss mode converters

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- Number: US 10,120,135 B2
- Document ID: US10120135B2
- Status: Granted
- Kind: B2
- Application number: 15/614,333
- Filed: 2017-06-05
- Published: 2017-09-21
- Granted: 2018-11-06
- Earliest claimed: 2014-04-30
- Assignees: Futurewei Technologies, Inc.
- Applicants: Futurewei Technologies, Inc.
- Inventors: Li Yang; Huapu Pan; Qianfan Xu; Dawei Zheng; Xiao Shen
- Web page: https://patentgrep.com/patent/US10120135B2

## CPC

- G02B 6/14: Mode converters
- G02B 6/1228: {Tapered waveguides, e.g. integrated spot-size transformers (for coupling with fibres G02B6/305)}
- G02B 6/132: by deposition of thin films
- G02B 6/136: by etching
- G02B 6/305: {and having an integrated mode-size expanding section, e.g. tapered waveguide}
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

An apparatus comprises a substrate comprising a silicon dioxide (SiO2) material disposed on top of the substrate, a silicon waveguide comprising a first adiabatic tapering and enclosed in the silicon dioxide material, and a low-index waveguide disposed on top of the substrate and adjacent to the first adiabatic tapering. A mode converter fabrication method comprises obtaining a mode converter comprising a substrate, a silicon waveguide disposed on the substrate and comprising a sidewall and a first adiabatic tapering, and a hard mask disposed on the silicon waveguide and comprising a silicon dioxide layer, wherein the hard mask does not cover the sidewall, and oxidizing the silicon waveguide and the hard mask, wherein oxidizing the silicon waveguide and the hard mask encloses the silicon waveguide within the silicon dioxide layer.

## Claims

### Claim 1

A mode converter fabrication method comprising:
obtaining a mode converter comprising:
a substrate,
a silicon waveguide disposed on the substrate and comprising a sidewall and a first adiabatic tapering, and
a hard mask disposed on the silicon waveguide and comprising a silicon dioxide (SiO2) layer, wherein the hard mask does not cover the sidewall; and
oxidizing the silicon waveguide and the hard mask to enclose the silicon waveguide within the silicon dioxide layer.

### Claim 2 (depends on 1)

The method of claim 1, wherein the substrate comprises a second silicon dioxide layer.

### Claim 3 (depends on 1)

The method of claim 1, wherein the hard mask further comprises a silicon nitride (Si3N4) layer disposed on top of the silicon dioxide layer.

### Claim 4 (depends on 1)

The method of claim 1, wherein oxidizing the silicon waveguide and the silicon dioxide layer comprises using dry thermal oxidation.

### Claim 5 (depends on 1)

The method of claim 1, further comprising reducing a tip width of the silicon waveguide via the oxidizing.

### Claim 6 (depends on 1)

The method of claim 1, further comprising patterning onto the substrate a second waveguide adjacent to the silicon waveguide.

### Claim 7 (depends on 6)

The method of claim 6, wherein the second waveguide is in direct contact with the silicon dioxide layer.

### Claim 8 (depends on 6)

The method of claim 6, wherein the second waveguide comprises a second adiabatic tapering.

### Claim 9 (depends on 8)

The method of claim 8, wherein at least a first portion of the first adiabatic tapering and at least a second portion of the second adiabatic tapering abut each other.

### Claim 10 (depends on 8)

The method of claim 8, wherein a first width of the first adiabatic tapering is widest at a first location along the substrate, and wherein a second width of the second adiabatic tapering is narrowest at the first location.

### Claim 11 (depends on 10)

The method of claim 10, wherein the first width is narrowest at a second location along the substrate, and wherein the second width is widest at the second location.

### Claim 12 (depends on 11)

The method of claim 11, wherein the first width is greater than 0.4 micrometers (μm) at the first location and is between about 50 nanometers (nm) and 60 nm at the second location, and wherein the second width is about 1 μm at the first location and is 15 μm or greater at the second location.

### Claim 13 (depends on 12)

The method of claim 12, wherein a first thickness of the silicon waveguide is about 0.18 μm at the first location and about 0.15 μm at the second location, and wherein a second thickness of the silicon waveguide is between about 1 μm and about 15 μm at the first location and the second location.

### Claim 14 (depends on 1)

The method of claim 1, further comprising further oxidizing the silicon waveguide and the hard mask to fully enclose the silicon waveguide within the silicon dioxide layer.

### Claim 15

A mode converter fabrication method comprising:
fabricating onto a substrate a silicon waveguide that comprises a first adiabatic tapering and a sidewall, wherein a hard mask is disposed on the silicon waveguide and does not cover the sidewall, and wherein the hard mask comprises silicon dioxide (SiO2) material;
fabricating a second waveguide onto the substrate, wherein the second waveguide comprises a second hard mask enclosing the second waveguide; and
oxidizing the silicon waveguide and the second waveguide until the silicon waveguide is enclosed within the silicon dioxide material.

### Claim 16 (depends on 15)

The method of claim 15, further comprising reducing a tip width of the silicon waveguide via the oxidizing.

### Claim 17 (depends on 15)

The method of claim 15, wherein the second waveguide comprises a second adiabatic tapering.

### Claim 18 (depends on 17)

The method of claim 17, wherein at least a first portion of the first adiabatic tapering and at least a second portion of the second adiabatic tapering abut each other.

### Claim 19 (depends on 17)

The method of claim 17, wherein a first width of the first adiabatic tapering is widest at a first location along the substrate and narrowest at a second location along the substrate, wherein a second width of the second adiabatic tapering is narrowest at the first location and widest at the second location.

### Claim 20 (depends on 15)

The method of claim 15, wherein the silicon waveguide is separated from the second waveguide by a low-index material, and wherein the substrate comprises a silicon (Si) layer and a buried oxide (BOX) layer.