Optiwave offers tools for modelling waveguides, fibers, optical systems and electronics, while Lambda Research's solutions — TracePro and OSLO in particular — complete this process on the side of classical optics, ray tracing, and complete opto-mechanical geometry.
What can an optical sensor measure?
A change in the optical signal can be caused by a wide range of physical quantities:
- temperature;
- strain;
- pressure;
- displacement;
- vibration;
- concentration of a substance;
- the surrounding medium's refractive index;
- the presence of specific chemical compounds;
- magnetic field;
- motion.
Optiwave points to applications of optical sensors in electronics, biomedicine and other fields, and presents examples of PCF sensors and Bragg-grating-based sensors.
What matters most is the quantity that changes in the simulation
Sensor design should start with the question: how does the measured quantity affect the light? Temperature, for example, can change the refractive index, the length of an element, the resonance condition, or the position of a maximum or minimum in the spectrum. In a refractometric sensor, a change in the surrounding medium's refractive index can change the mode's effective refractive index. In an FBG sensor, strain or temperature can shift the spectral characteristic. Only once this mechanism has been identified can the right model be chosen.
Sensor sensitivity
One of the basic parameters is sensitivity: S = Δy / Δx, where x is the measured quantity and y is the observed optical parameter. If the measured quantity is a change in refractive index and the observed quantity is a shift in the resonance wavelength, you can analyse S = Δλ / Δn.
High sensitivity alone does not guarantee a good sensor, though. Just as important are:
- measurement range;
- linearity;
- resonance width;
- stability;
- signal-to-noise ratio;
- temperature sensitivity;
- manufacturing tolerances;
- the ability to unambiguously read out a change.
Fiber-optic sensors
Optical fibers are a natural platform for building many kinds of sensors. Depending on the construction, the measured change can affect the propagating mode, polarization, phase, loss, reflection, or the resonance wavelength. Fiber parameters themselves can be designed with OptiFiber or OptiMode, while system-level analysis of the measurement path uses OptiSystem.
FBG sensors
One of the most important classes of fiber-optic industrial sensors uses Fiber Bragg Gratings. A Bragg grating has a characteristic spectral response whose position can shift under physical conditions acting on the structure — such a solution can be used, among other things, to measure temperature or strain. Optiwave publishes materials on designing FBG sensors, and a webinar on sensor design in OptiSystem covers Bragg-grating sensors among other topics.
PCF sensors
Another example is Photonic Crystal Fibers. In structures of this type, the field distribution can be very sensitive to the hole geometry and to the properties of the material surrounding them. Optiwave presents design examples for a temperature sensor based on PCF and a biochemical PCF sensor. In the biochemical example, OptiMode is used to model the structure's optical properties, analyse different geometries and materials, and visualize electric and magnetic fields. This is a typical case where the sensor's function arises directly from a change in the mode's properties.
From a component to a complete measurement path
A component model answers the question "how does its optical response change?". It doesn't yet answer the question "can that change be measured reliably?". That is why a sensor should also be analysed at the system level. Such a model can include: source → sensor → optical path → photodetector → signal analysis. In OptiSystem you can build complete optical systems and run parameter sweeps, letting you study how changes in the sensor affect the signal observed at the receiver.
The role of electronics
In a practical sensor, the optical signal has to be converted into an electrical one. This brings in photodiodes, transimpedance amplifiers, filters, threshold circuits, source control, and feedback. If you need to analyse the optical and electronic domains together, you can use OptiSPICE — the program lets you simulate optical and electrical circuits in one environment, including cases where feedback crosses between the two domains.
TracePro — when the sensor's complete geometry has to be taken into account
In many sensors, the problem doesn't end at propagation inside the fiber. The signal may pass through a lens, a protective window, a housing, the surface of the object being measured, a filter, a mechanical element, and a detector — parasitic reflections and stray light appear. In such cases, TracePro from Lambda Research is very useful. TracePro uses non-sequential ray tracing and can model light propagation through the complete opto-mechanical geometry, accounting for scattering and unwanted light paths, among other effects.
An industrial sensor has to work outside the lab
In industrial conditions a sensor can be exposed to sunlight, light from machinery and lamps, glare from shiny surfaces, dust, changes in position, vibration, temperature, and assembly tolerances. A good design therefore cannot assume a perfectly aligned optical axis and a perfectly black housing. In analysing such problems, TracePro can simulate complete mechanical parts, coatings and materials, and then identify unwanted ray paths. Lambda Research describes analogous problems in the context of industrial and automotive optical sensors, where stray light, reflections and environmental conditions can degrade signal integrity.
OSLO — designing the imaging path
If a sensor relies on imaging, lens parameters and image quality become important. In that case, the natural complement to TracePro is OSLO. OSLO is used to design and optimize optical systems, while TracePro can later analyse the actual system geometry, stray light, and interaction with mechanical elements. A typical workflow can therefore look like: OSLO — lens system → TracePro — full geometry and stray light → OptiSystem / OptiSPICE — signal path and electronics.
Virtual disturbance testing
One of simulation's biggest advantages is the ability to study cases that are hard to systematically reproduce during early prototyping. You can analyse a shift in the source, a change in angle, contamination of a surface described by an appropriate model, a change in reflectance, an additional light source, a change in the properties of the medium being measured, a change in temperature, or a tolerance in detector position. This way, the designer doesn't get a single sensitivity value, but a map of the device's behaviour over the expected operating range.
Validating the real sensor
The final stage is always the experiment. Once a prototype has been built, its characteristics can be measured automatically using lab equipment controlled by OptiInstrument. The full process then looks like: component model → system model → opto-mechanical model → prototype → automated measurement → validation. This is especially important for sensors, where the real sensitivity depends on several phenomena at once.
Summary
There is no single simulation method suited to all optical sensors. If the measurement information arises from a change in the mode or waveguide structure, you need a wave-level model. If a Bragg grating is used, you need a model of its spectral response. If the whole signal path needs to be assessed, you need a system-level simulation. If the sensor includes electronics, you need an optoelectronic model. If lenses, housings, reflections and stray light are the key factors, you need ray tracing. That's why a practical industrial sensor design often draws on several environments: OptiMode / OptiFiber / OptiGrating → OptiSystem → OptiSPICE → OSLO / TracePro → OptiInstrument.
Manufacturer materials and illustrations
Optiwave's set of sensing examples, including illustrations of PCF sensors and FBG materials: Optiwave – Optical Sensing
A biochemical PCF sensor example with geometry and FEM mesh: Optiwave – Photonic Crystal Fiber Bio-chemical Sensor
A manufacturer webinar on sensor design in OptiSystem, covering LiDAR, fiber-optic gyroscopes and FBGs, among other topics: Optiwave – Optical Sensor Design using OptiSystem
Lambda Research material on analysing automotive optical sensors: Lambda Research – Optical Analysis of Automotive Sensors