The next generation in photonic layout & simulation

1. The next generation in photonic layout & simulation “propagating ideas”

2. What is OmniSim ? • Truly direction agnostic layout and simulation tool • Time domain engine • 2D & 3D FDTD • generate spectral response in a single simulation • Frequency domain engine • new algorithm brings unparalleled speed • Automatic optimisation • evolves your design while you eat lunch ! • Mask export using GDSII “propagating ideas”

3. OmniSim Features Summary • OmniSim contains a sophisticated layout editor • Complex 3D devices can be built up using physical layers and process concepts • Elements can be rotated and linked to each other • A wide range of sources and sensors are available • Can import a FIMMWAVE mode for launching or measuring mode power – substantially increases simulation efficiency. • The frequency domain method offers a rapid technique for simulating devices • The FDTD technique is able to simulate either at CW or over a broadband • Kallistos can be used to optimise a device • The final design can be exported as GDSII data

4. OmniSim FDTD Engine Features • General • The fastest FDTD engine in the field • Sub-gridding – substantially reduce memory and time required c.f. variable grid FDTDs. • Many memory reducing technologies • Cluster system for distributing simulation across multiple CPUs/PCs • Materials • Anisotropy – general symmetric tensor. • Dispersive materials including metals – Drude/Debye/Lorentzian models with automatic fitting tool (you just provide a dispersion curve). • Chi2 and Chi3 non-linearity • Permeability (magnetic materials) • Boundary Conditions • High performance PMLs on 6 faces • Dispersive PMLs – e.g. continue a metal layer into the PML • Metal, magnetic and periodic boundary conditions

5. Sub-gridding • Photon Design first to demonstrate an efficient, stable sub-grid algorithm in FDTD – all previous published algorithms not stable. • Increase local resolution by 2x, 4x, 8x, … • Can accelerate a 3D simulation by 64x or more. • Much more efficient than “variable grid” FDTDs – a diagonal of small features would create a fine grid everywhere.

6. Mode Excitors and Sensors mode sensor mode excitor • Import precise mode from FIMMWAVE for mode launch • Ditto import for mode sensor • => Substantially decrease simulation volume/time. mode computed in FIMMWAVE

7. Example - ring resonator design/simulation • Editor allows very easy layout of even the most complex ring structures • Simulation with time or frequency domain “propagating ideas”

8. The Mask Editor • Free rotation of almost all objects • Powerful constraint system • Hierarchical framework • n-level undo/redo • Supports etch and growth processes • Export to mask file

9. Layout - SOI grating example First define the physical layers Define the device process steps, such as etch and growth

10. Layout - SOI grating example Use the physical layers and process steps to build up the device Use arrays of subelements for periodic structures Add viewpoints to confirm structure is as expected

11. Layout - SOI grating example Use viewpoints to confirm structure is as expected

12. Layout - Coupler Example Constraints link objects together Can now adjust waveguide separation just by moving a single lower waveguide Final design can be exported as GDSII

13. OmniSim - FD • A new powerful state of the art 2D Maxwell solver for propagation of EM fields within an arbitrary photonic structure. • Combines the problem solving capability of FDTD with the speed and accuracy of the frequency domain • Exceptionally fast • Order of magnitude faster than competing tools. • Based on new efficient numerical techniques • High delta-n capability • Integrated with OmniSim-TD engine • Wide range of sources and sensors • OmniSim-FD's speed and low numerical noise make it ideal for automatic optimisation

14. OmniSim - Design System • Take an initial design • Use Kallistos and the frequency domain for multi wavelength optimisation • Fine tune the device using 3D FDTD to produce an optimised design

15. Why use FDTD • FDTD is accurate and robust. • The sources of error in FDTD calculations are well understood • FDTD can simulate light travelling at any angle • Use FDTD where BPM and EME fail • FDTD treats impulse response readily. • FDTD directly calculates the impulse response of an electromagnetic system • A single simulation can provide the steady state response at any frequency within the excitation spectrum • FDTD accounts for all reflections • Transmission and reflection can be calculated in a single simulation • Computer memory capacities are increasing rapidly. • FDTD discretizes space over a volume, inherently needs large amounts of RAM

16. What is FDTD FDTD is direct discretisation of Maxwell’s Equation In 1D Maxwell gives us: Discrete form gives, for a space and time interval dx, dt:

17. Some FDTD guidelines • FDTD gives spectral response over a wide band, by Fourier Analysis. • The timestep dt is approximately dx/c • Run a simulation for time tTot with timestep dt, then • Spectral resolution: df = 1/ tTot • Spectral range: fTot = 1/dt • Structures with small features  fine grid  small timestep • For high spectral resolution the simulation requires many timesteps. • FDTD uses lots of memory • The algorithms used in OmniSim are very memory efficient • Mirror planes have been introduced to reduce memory requirements • New method to reduce memory requirements by upto a factor of 8 or 16

18. Spectral width of source The spectral width of the source is given by Fourier analysis A CW source transforms to a delta function i.e. it has zero spectral width To look at a broad spectral response we need to launch a short pulse A finite pulse transforms to a finite spectral width If T= pulse length then the spectral width F is given by 1/ T In wavelength terms =-2/cT For example a 20 femtosecond pulse at 1.55 um has a spectral width of 700 nm

19. OmniSim - FDTD • A highly efficient FDTD (finite difference time domain) engine • 2D and 3D FDTD engine • very fast speed optimised algorithm • More memory efficient than competing products • PML and metal boundaries • Many optical sources available including waveguide mode and dipole • Run time monitoring of evolving fields • Wavelength response spectra • Complex refractive index • Material database uses Debye, Drude or Lorentzian model to automatically fit to a wide range of metals Right: FDTD simulation of optical pulses scattering off cylindrical objects

20. Corner reflector example Define a corner mirror using different mask layers

21. Corner reflector example Silica waveguides have too low a refractive index for a 90 degree corner reflector But the reflector works quite well for 60 degrees !

22. Coupler design example A 20 fs sinusoidal pulse centred at 1.55 um wavelength Contains spectral information over 700 nm Wavelength response of device

23. Coupler design example Contains spectral information over 800 nm Use CW at 1.55 um wavelength to study the field distribution

24. Ring resonator design example Use coupler as building block for ring resonator

25. Simulation time comparisons Contains spectral information over 800 nm The frequency domain method is very fast and unique to Omnisim For some devices it might be better to do multiple frequency domain simulations rather than a single broadband FDTD

26. Ring resonator design example Use coupler as building block for ring resonator

27. Kallistos - automatic optimisation • Kallistos can evolve designs that are simply not possible by traditional techniques. • Choice of sophisticated optimisation algorithms - local and global • Objective function (determines “good” and “bad”) can be any expression • Can optimise any 3-5 parameters globally • Can optimise any 30+ parameters locally • Multi-wavelength optimisation to maximise device bandwidth

28. Kallistos - Ring resonator example • The wavelength peaks are determined by the length around the ring • insert variable straight length L between the two arcs • The extinction ratio is determined by the coupler • Vary the gap between the waveguides • Set up optimisation example to tune resonator transmission to 1.7 um and maximise the extinction ratio

29. Kallistos - Ring resonator example • Define two independent variables • Length will represent the length of waveguide between the arcs • Gap will represent the edge to edge separation between the arms of the couplers

30. Kallistos - Ring resonator example • Link the independent variables to the structure • Commands can be found very easily by pressing the tab key

31. Kallistos - Ring resonator example • Define an objective function • In this example • the wavelength is set to 1.7 • an FD calculation is performed • the flux in the through and the drop arm is calculated • the wavelength is set to 1.72 um • the calculation is repeated • The objective is defined as the extinction ratio at the two wavelengths which is then to be maximised • The resonator will be tuned to 1.7 um and the free spectral range set to 40 nm

32. Kallistos - Ring resonator example • Run the optimisation • Using global optimisation • Several peaks are found corresponding to different spectral orders

33. Kallistos - Ring resonator after optimisation • Transmission at 1.7 um • Transmission at 1.72 um

34. Active FDTD Engine • Model gain in FDTD simulation • Includes a rate-equation for carrier dynamics • Multi-Lorentzian gain model (first ever in World) • User-defined “contacts” for current-injection • Applications: • Photonic crystal lasers • Nano-cavity lasers Right: a Littrow mode of a photonic crystal laser simulated with CrystalWave.

35. Conclusions • OmniSim provides one of the easiest to use interfaces in the industry, with powerful features such as tilted etching and grading. • Advanced fast FDTD engine boasts many World-first technologies including sub-gridding and realistic gain materials