Solutions | Optical Systems

The optical sensor design workflow

Turn a physical or chemical quantity into a change in light, and then into a signal you can read out. Integrated waveguide photonics is one strong implementation; free-space optics, imaging and detection systems, resonant structures and plasmonic structures serve the same purpose. What sets the detection limit is not sensitivity itself but how much of that sensitivity is eaten by noise, thermal drift, optical loss and assembly misalignment.

Design challenges

Designing a small perturbation as a detection limit

A sensor signal is a perturbation. In a waveguide sensor, for instance, the region where a target bound to the surface changes the refractive index is only tens of nanometers thick. That is why the optical field has to be placed deliberately where the quantity you want to measure is. The principle is the same for free-space systems and resonant structures. That same high sensitivity applies equally to noise, temperature, optical loss and assembly misalignment, so the detection limit accumulates as the ratio of response to competing factors, not as the size of the response.

Requirement | transduction sensitivityThe optical field is placed where the measured quantity is, and quantified as response per unit of that quantity

Requirement | building up the detection limitElement sensitivity, noise, thermal drift, optical loss, the readout scheme and assembly tolerances are accumulated into an estimate

Requirement | choosing the method for the architectureThe analysis method is chosen according to whether the architecture is waveguide, free-space, or resonant and plasmonic

Design and simulation workflow

From choosing the transduction scheme to verifying the assembly

Choose the transduction scheme and architecture

Choose an architecture whose field overlaps the measured quantity, including whether it is waveguide-based or free-space.

Extract the perturbation

Sweep the measured quantity and obtain the change in refractive index or optical path and its wavelength dependence.

Design the resonant or interferometric structure

Design a structure that converts the index change into a change in intensity or wavelength.

Evaluate the sensor circuit and readout

Settle the operating point and signal processing conditions from the response curve and the readout scheme.

Verify noise, temperature and detection

Confirm the operating point with temperature dependence and noise included, and obtain the responsivity of the photodetector.

Verify assembly and the spectrometer path

Evaluate coupling misalignment at assembly, and for a spectrometer path confirm tolerances and stray light.

The stages differ by project. The analysis methods used change with the architecture, and MPW or a dedicated run applies only where the device is built as integrated photonics.

Related analysis technologies

Analysis methods behind each step

The principles and scope of each method are explained on the Analysis Technologies pages.

FDE (Finite-Difference Eigenmode)Obtain the guided mode and the change in effective index with the measured quantity (stage 02)About FDE →FEEM (Finite-Element Eigenmode)Solve modes of curved or complex cross-sections by finite elements (stage 02)About FEEM →FDTD (Finite-Difference Time-Domain)Solve resonant and plasmonic structures rigorously in three dimensions (stage 03)About FDTD →RCWA (Rigorous Coupled-Wave Analysis)Obtain the response of a periodic structure across a sweep of wavelength and angle (stage 03)About RCWA →CHARGE (Drift-Diffusion Analysis)Obtain photodetector responsivity and dark current from carrier transport (stage 05)About CHARGE →HEAT (Thermal Conduction Analysis)Obtain thermal drift and evaluate it as movement of the operating point (stage 05)About HEAT →Stray Light AnalysisQuantify light leaking around the spectrometer path (stage 06)About stray light analysis →

Products used at each step

The capabilities and coverage of each product are described on its own product page.

Lumerical MODE

Handles guided mode and perturbation analysis. FDE gives the change in effective index with the measured quantity and its wavelength dependence.

Stage / 02

View Lumerical MODE

Lumerical FDTD

Handles analysis of resonant and plasmonic structures. Three-dimensional FDTD and RCWA give the response against wavelength and angle.

Stage / 03

View Lumerical FDTD

Lumerical Multiphysics

Handles finite-element analysis of the cross-section, thermal drift and the photodetector. FEEM, HEAT and CHARGE are used according to the architecture.

Stage / 02, 05

View Lumerical Multiphysics

Lumerical INTERCONNECT

Handles evaluation of the sensor circuit and readout. The response curve and readout scheme settle the operating point and signal processing conditions.

Stage / 04

View Lumerical INTERCONNECT

Zemax OpticStudio

Handles verification of coupling and the spectrometer path. Physical optics propagation evaluates assembly misalignment, and stray light analysis quantifies unwanted light in the path.

Stage / 06

View Zemax OpticStudio

Foundry and manufacturing

Connecting design data to manufacturing

Where the device is built as integrated photonics, once the layout is settled you move to prototyping through a multi-project wafer (MPW) or a dedicated run. Where a free-space, resonant or plasmonic structure is built another way, this stage does not apply.

Whether prototyping and manufacturing are possible, and in what form, is judged case by case from the architecture chosen and the process conditions.

View foundry services

Related technical information

Talk to us about what you are measuring and your target detection limit

Tell us where the design stands, how far architecture selection has progressed, whether you have measured data, and your plans for readout and assembly, and we will propose a way forward.

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