My Research and Inventions

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Introduction

Si nanowire fabrication technology

fiber/waveguide coupling setup

devices made of Si-waveguides

directional coupler

integration: plasmonic + Si waveguides

two technology of integration

Out-plane plasmonic confinement

plasmonic isolator

III-V plasmonic

AlGaAs waveguide (800 nm)

AlGaAs waveguide (1550 nm)

Si wire waveguide (1550 nm)

plasmonic on GaAs (800 nm)

plasmonic on GaAs (1550 nm)

plasmonic on Si (1550 nm)

plasmons in metal strip

in-plane confinement of plasmons

samples very old

 

 
Directional Coupler

Directional coupler made of Si wire waveguides (wavelength=1550 nm)

Fabrication technology of directional coupler is here and here

 

 

Fig.1 Conversion of TM mode of two parallel waveguides. Gap between waveguides is 250 nm. Thickness of Si waveguides 220 nm, width 450 nm. The conversion length of light from one waveguide to another is 6.7 um (See Fig.2)

 

It is important component of many devices made of Si-nanowire waveguide. Depending on device, a different coupling ratio is required:

1) polarizer and polarization beam splitter

100 % for TM mode, small for TE mode

2) Mach–Zehnder interferometer

50 %

3) ring resonator

small

 

 

 

 

 

 

 

 

comsol+ matlab files for calculation can be downloaded here

coupler matlab comsol.zip

DirectionalCoupler2.m DirectionalCoupler2.mph: Calculation for a fixed gap
DirectionalCoupler_scan.m: scan over range of different gaps + animation

 

 

 

 

 

Case 1: Si nano waveguides are not covered by SiO2

Optical field of TE mode is mainly inside the Si wire. Significant amount of optical field of TM mode is inside the SiO2 substrate.

Fig..2 The length of full conversion of waveguide mode from a waveguide to a waveguide as function of gap between two parallel Si waveguides. The waveguides are not covered by SiO2. Thickness of Si waveguides 220 nm, width 450 nm.

 Fig.2a Conversion of TE mode of two parallel waveguides. Gap between waveguides is 250 nm. Thickness of Si waveguides 220 nm, width 450 nm. The conversion length of light from one waveguide to another is 68 um (See Fig.2)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Case 2: Si nano waveguides are covered by SiO2

 

Fig..3 The length of full conversion of waveguide mode from a waveguide to a waveguide as function of gap between two parallel Si waveguides. The waveguides are covered by SiO2. Thickness of waveguides 220 nm, width 450 nm.

 Fig.3a Conversion of TM mode of two parallel waveguides. Gap between waveguides is 450 nm. Thickness of Si waveguides 220 nm, width 450 nm. The conversion length of light from one waveguide to another is 15 um (See Fig.2)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Directional coupler as a polarization splitter or a polarizer

 

Directional coupler as a polarizer

Fig.5 Maximum polarization ratio, which can be achieved by a directional coupler as function of gap in the coupler. For each gap the length of the coupler is adjusted to 100 % of TM mode conversion (See red line in Fig.2 and 3). The distance of The width of Si waveguides is 450 nm

 

 

The fabrication technology of polarizer made of Si-nanowire waveguides is here

 

It is relatively easy to make a polarizer of polarization beam splitter using a directional coupler.

As can be seen from Fig.2 and 3, the distance for 100% conversion is significantly different for TM and TE modes. For example, from Fig 3 for 400 nm gap the distance for 100% conversion is ~11 um for TM mode and ~150 um for TE mode. In case of length of coupler of 11 um, 100 % of TM polarization is converted into output 2, but there is only a very little conversion of TE polarization and near all TE polarized light goes to output 1. Therefore, the coupler splits TM-polarized light to output 2 and TE-polarized light to output 1.

From calculation of Fig.5, a larger polarization ratio can be achieved in coupler with larger gap between the coupling waveguides. From Fig.2 and 3 the larger gap requires a longer interaction distance for required 100 % of conversion of TM mode.