Optical simulation of light delivery system is important to design an optically efficient HAMR device. Using a combination of FDTD and MODE, we can perform the optical system simulation and can evaluate the optical efficiency of the HAMR system. In this page, we'll explain the steps how to perform the system simulation.
The figure below shows the schematic of HAMR light delivery system here we consider. A laser light with a wavelength 650nm illuminates the waveguide inlet and the transmitted light travels within a waveguide with tapered section at the end. The length of the waveguide in HAMR system is usually more than a hundreds of micrometer and we need a large scale light propagating simulation. Here, we use 2.5D propagator in MODE and perform the light propagation over a 100 μm. At the end of the waveguide, i.e. at the tapered section, the light is squeezed into a small region and hits a transducer part with a plasmonic particle. By the excitation of localized surface plasmon, nanoscale strong light localization can be achieved and the localized light heats a region in the data layer, resulting in a nanoscale data recording.
The light delivery system simulation using Lumerical's optical stimulation tool can be performed as follows.
Step 1. Calculate or Import custom laser profile into FDTD and perform far field projection to obtain an input beam profile at the waveguide inlet.
Step 2. Using FDTD and/or MODE, overlap and/or coupling calculation between the calculated input beam profile in the previous step and the mode profile of the waveguide is performed
Step 3. Using 2.5D Propagator in MODE, a large scale light propagation within the waveguide is performed and a field profile at the end facet of the tapered section is calculated.
Step 4. Importing the field profile calculated in Step 3 into FDTD, nano scale focusing at the plasmon transducer part is preformed. In this step, a field data recorded on a profile monitor data in step 3 is imported in to FDTD. Regarding with how to export data to another simulation, please see the Custom field profile from monitor data page.
The associated files you can download in the above box are examples of light delivery system simulation. Each simulation file can be used as follows.
Step 1 : Custom laser profile calculation and a far field projection
In the script file HAMR_light_delivery_custom_source.lsf, an analytic Gaussian beam is used as a custom laser profile. Run this script file with usr_create_fld.lsf, then you obtain fld data file named source.fld which is used as a source profile for a far field projection. Next, Import the calculated laser field profile and perform far field projection. In a custom source object in HAMR_far_field_projection.fsp, the calculated laser field is already imported. If you run script file HAMR_far_field_projection.lsf after completing the HAMR_far_field_projection.fsp simulation, you obtain HAMR_far_field_source.fld data file where the far field profile at 1.5 mm away from the lase source is recorded. This profile can be used to calculate coupling with waveguide modes in the next step.
Step 2 : Overlap or Coupling calculation between input beam profile and waveguide mode
In a custom source in HAMR_mode_expansion.fsp, the field profile obtained in the previous section is imported and you can calculate power coupling. Regarding with the details of coupling calculation, see using mode expansion monitors page. Also, if you calculate mode profile on HAMR_overlap.lms and import ff_source.fld in the deck field in the eigensolver analysis window, you can perform overlap calculation. between the inpot fie and a modeprofie of the wavegiude. See the overlap page for more information on mode overlap calculations.
Step 3 : Light propagation in waveguide
HAMR_waveguide_propagation.lms simulates light propagation in the waveguide. After completing the simulation, run script file HAMR_profile_data_extract.lsf, then you obtain data file usr_create.fld with is used as input source profile in nanofocusing simulation in the next step.
Step 4 : Nano scale focusing simulation
In a custom source object in HAMR_transducer.fsp, the field profile obtained in the previous setp is imported. Run this HAMR_transducer.fsp, then you can estimate nanofocusing effect of light.
Tips and additional methodology
Measuring absorption per unit volume
For more information about measuring absorption per unit volume, see the Absorption per unit volume page. For HAMR systems, the 'advanced' version of the Pabs analysis object is definitely recommended.
Using simulation results as a source in another simulation
When simulating the entire light delivery system, it is sometimes necessary to use the simulation results from one simulation (eg. the taper) as the source field profile in a second simulation (eg. the transducer). This can be accomplished with the Import source. See the Custom field profile from monitor data page for details.
More plasmonic examples
For more information on simulating plasmonic devices (often used for transducers in HAMR systems), see the 'Plasmonics' categoriy of the Application Gallery.
Modeling extremely thin layers
Layers near the transducer and data recording region can be extremely thin (a few nm's). Such thin layers are challenging to simulate because a very small mesh size is required to resolve those layers, which leads to very long simulation times and high memory requirements. We recommend running your initial simulations using a larger mesh (eg. 10nm, 5nm, 2nm), even if that means some layers will be excluded from the simulation. Using a large mesh will make the simulation much faster, which allows you to ensure the basic simulation setup is correct without taking too much time. Once the basic simulation setup is correct, you can begin to use a smaller mesh size to more accurately resolve the layers.
Tip: Mesh override regions can be used to force a smaller mesh size. In many cases, a smaller mesh is only required in the Z direction.
More large planar waveguides and taper examples
For more information on simulating large waveguides and tapers used for light delivery, see the 'Photonic integrated circuits - Passives' category of the Application Gallery.
Thermal simulations of HAMR devices is another critical step in the overall HAMR device simulation flow. The absorption per unit volume data must be exported from the optical simulation (FDTD) to the thermal solver. For example, the matlabsave command can be used to export the entire Pabs dataset into a Matlab data file. Your thermal solver can then import the absorption data from the .mat file. It will also be necessary to interpolate the absorption data from the cartesian mesh used in the FDTD simulation onto the (typically) finite element mesh used in the thermal simulation. It is usually most convenient to do this interpolation step in your thermal solver.
- K. Takano, E. Jin, T. Maletzky, E. Schreck, and M. Dovek, "Optical Design Challenges of Thermally Assisted Magnetic Recording Heads," IEEE Trans. Magn., vol. 46, pp. 744–7507 (2010).