Optical Waveguide

Introduction

● Existing technology

○ Much existing research has been done on silicon waveguides

○ Optical fibre is an example of silicon waveguide technology

● Problem

○ Transparency of silicon lies below 8µm

○ Germanium with a transparency range of 1.6µm to 18µm is more suitable

Objective

Determine the cut off dimensions of Single-mode

Germanium on Silicon optical planar waveguide in mid-IR

region

Total Internal Reflection

● For total internal reflection to occur, the propagating wave must be travelling

in a medium of higher refractive index and hit a boundary with a lower

refractive index.

● If angle of incidence > critical angle, total internal reflection will occur

The critical angle can be derived by equating the angle of refraction to be 90.

𝑛1𝑠𝑖𝑛𝜃𝑖 = 𝑛2𝑠𝑖𝑛𝜃𝑟

𝑠𝑖𝑛𝜃𝑖 =𝑛2𝑛1

𝑠𝑖𝑛𝜃𝑟

Methodology

● Ran simulations with the software MODE Solutions by Lumerical

Parameters

● Wavelength of source: 3.8 µm

Apart from the wavelength falling in the Mid-IR region, the actual laser source

that is used in the lab now is a 3.8 µm laser.

● Germanium core

○ Transparency: 1.6 µm to 18 µm, suitable in the mid-IR region (2-20 µm).

○ Refractive index of 4.03, higher than the silicon cladding with refractive

index of 3.42.

○ Higher electron mobility at 3900cm2V−1s−1 compared to silicon’s

1400cm2V−1s−1

Parameters

● Initial dimensions of germanium core:

○ x span = 50 µm, y span = 2 µm, z span = 1.5 µm

● Dimensions of substrate:

○ x span = 50 µm, y span = 50 µm, z span = 10 µm

Methodology

● Obtain the modal analysis for the first four modes.

○ Modes 1&2 (TE0, TM0) – Single Mode propagation

○ Modes 3&4 (TE1, TM1) – Dual Mode propagation

● Sweep the y and z span

○ Obtain graph with y axis as effective refractive index and x axis as the y

span and z span in two separate graphs

● Determine the dimensions with the sweep results

Results

Results

● Modal analysis

● Ensure that first 2 modes are single mode, next 2 are dual

Mode 1 Mode 2

Results

Mode 3 Mode 4

Results

● Vary the y- and z-spans

● Obtain effective refractive index, neff, from the simulation

● Calculate cutoff dimension where neff > nSi

Results

Y-span Z-span

3.42

Results

3.423.42

Z-span

Results

● Cut-off dimensions obtained: 50µm by 2µm by 0.6µm

● Zhang.H. previously obtained values of 2 μm and 1.38μm (y and z)

● Our results differ by 0.78μm in the z-span

● Accounts for Zhang’s high transmission loss

● Reduced loss

Conclusion

Conclusion

● Germanium has many applications in chemical sensing and environment

monitoring

● We found out the cutoff dimensions for a single-mode Ge waveguide

Further Applications

● Detection of specific chemicals

● Optimising the waveguide dimensions to reduce loss