Post on 10-Oct-2020
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