PIC : the photonic integrated circuit. What is it and which tools are used to design it?

In a PIC, the width and height of a waveguide affect the modes, the material sets the refractive index, the spacing between waveguides governs the coupling, and the difference in optical path lengths can set the phase of an interfering signal. Depending on the problem, you may need mode analysis, BPM propagation, a full FDTD electromagnetic analysis, a compact model, an optoelectronic-circuit simulation, or a whole-system simulation.

In the Optiwave ecosystem, this is handled mainly by OptiMode, OptiBPM, OptiFDTD, OptiOmega, OptiSPICE, and OptiSystem. Lambda Research — with OSLO and TracePro — supplements the project where the PIC meets a classical optical system, 3D geometry, and ray analysis. We cover the broader industrial context in the article Photonics in industry: simulation and design.

PIC : what is actually on the chip?

A photonic integrated circuit can contain, among others:

• optical waveguides;
• splitters and combiners;
• directional couplers;
• MMI (Multimode Interference) structures;
• MZI (Mach–Zehnder Interferometer) interferometers;
• ring resonators;
• filters, modulators, and optical switches;
• WDM structures;
• chip–fiber couplers.

From the designer's point of view, however, these are not independent „building blocks”. Each is a specific electromagnetic structure, and its behaviour depends on the geometry, materials, wavelength, polarisation, and input and output conditions. That is why the first design stage often concerns not the whole circuit, but a single waveguide.

OptiMode : first you have to know which modes the waveguide guides

A waveguide does not guide light in an arbitrary way. For a given geometry, materials, and wavelength there is a specific set of mode solutions. We are interested in, among others:

• the mode field distribution;
• the effective refractive index (n_eff);
• the properties of the individual modes;
• how the solution depends on the geometry and material;
• the polarisation behaviour.

This level of analysis is what OptiMode is for — a CAD environment for designing and doing mode analysis of waveguide structures, used to determine mode characteristics and analyse electromagnetic solutions in waveguide structures. OptiMode mainly answers the question of what mode solutions a given structure has — this is not yet an analysis of the whole PIC.

Example

We design a silicon waveguide for a given wavelength and change its width. We are not yet asking how the whole circuit will behave — we ask: is the waveguide single- or multi-mode? How does n_eff change? What does the field distribution look like? That is a mode problem — OptiMode.

OptiBPM : how does the field propagate through the device?

Once we know the waveguide's properties, the next question is: what will happen to the field as it propagates through the structure? This is where BPM (Beam Propagation Method) comes in. OptiBPM is a CAD environment for designing complex optical waveguides, covering guiding, coupling, switching, splitting, multiplexing, and demultiplexing of optical signals.

BPM lets you analyse the propagation of the field along a structure without running a full time-domain electromagnetic simulation of the whole geometry. This makes the method very useful for structures such as tapers, branches, couplers, switching elements, MMI structures, and other waveguide devices in which the dominant problem is propagation in a given direction. OptiBPM can be used in 2D and 3D analyses, depending on the configuration and solver.

OptiMode answers „which modes does my waveguide have?”, and OptiBPM — „how does the field propagate through the designed structure?”. These are not two programs solving the same problem.

OptiFDTD : when a fuller electromagnetic analysis is needed

Not every structure can be conveniently described with BPM. For more complex devices we are interested in direct solutions of Maxwell's equations in a given geometry — in that case you can use FDTD (Finite-Difference Time-Domain). OptiFDTD is an environment for designing and simulating advanced passive and nonlinear photonic components; the vendor lists applications including integrated photonics, filters and resonators, photonic crystals, plasmonic structures, and diffractive elements.

An important feature of FDTD is the ability to model phenomena for which simpler propagation approximations may be insufficient — reflections, diffraction, scattering, coupling, resonances, local electromagnetic effects, and more complex nanophotonic geometries. OptiFDTD also allows geometry exchange with external CAD tools, including via the GDSII format, and cooperation with OptiBPM. We describe the method itself in the article How does the FDTD method work, and when should you use it?.

OptiOmega : a new element of the PIC workflow

Optiwave's current portfolio also includes OptiOmega — an environment that combines a GPU-accelerated FDTD solver with a mode solver for designing and analysing photonic devices. The tool offers, among others, automatic port detection, placement of input planes and monitors, and calculation and export of S-parameters.

Particularly important is the ability to work with the other Optiwave tools: results such as the effective index or S-parameters can then be used to build PIC schematics in OptiSPICE and OptiSystem. This shows the direction of the modern workflow well: electromagnetic solver → device parameters → circuit model → system simulation.

Why not simulate the whole PIC with FDTD?

Suppose we have a PIC containing dozens or hundreds of elements. We may want to know the exact electromagnetic field at every point of the whole structure — but this is not always computationally reasonable. That is why PIC design is usually hierarchical.

First we analyse a single element in detail (e.g. FDTD → coupler) and obtain its characteristics. After that we no longer have to re-solve the coupler's whole electromagnetic geometry every time — we use its characteristics as a higher-level element model. In this way we move from a full field model to a device model, and then to a circuit model. This is what makes it possible to build larger PICs without running a full electromagnetic simulation of every element on every change of the whole circuit's parameters.

S-parameters : an important language for exchanging models

One way to describe a photonic device at a higher level is with S-parameters (scattering parameters). They are especially convenient when you want to treat a complex element as a block in a larger circuit: a coupler simulated in detail can be described by its port characteristics and then used as an element of a larger model.

Optiwave indicates the ability to import S-parameter files, including in the Touchstone format, into OptiSystem; the data can come from OptiBPM, OptiFDTD, or external tools. This is an important mechanism for scaling the model: we do not completely lose information about the device's behaviour, but we do not have to re-solve the full electromagnetic problem.

OptiSPICE : when the PIC meets electronics

A PIC very often does not work alone. A modulator may be driven electrically, a photodetector generates an electrical signal, a laser needs a driver, and a receiver may contain a transimpedance amplifier. So a problem arises: optics + electronics.

This level is what OptiSPICE is for — a tool for analysing integrated circuits that contain interactions between optical and electronic components. The vendor lists applications at the level of laser drivers, transimpedance amplifiers, optical interconnects, and electronic equalizers. The way the question is posed changes: we no longer ask „what is the field distribution in the coupler?”, but „how does the whole optoelectronic circuit respond to the input signal?”.

Optiwave also shows an OptiSPICE + OptiSystem co-simulation flow, in which an electrical circuit — e.g. a laser driver or a receiver circuit — can be integrated with the optical system.

OptiSystem : the whole-system level

Higher still is OptiSystem. Its job is not to replace an FDTD solver for a single nanophotonic element — OptiSystem is for designing and simulating optical systems, particularly at the transmission level. We can have the chain: laser → PIC modulator → channel → PIC receiver → photodetector → electronics, and analyse the behaviour of the whole link.

In such an environment we are interested in system parameters: time waveforms, frequency responses, the eye diagram, BER, Q-factor, optical power, OSNR. OptiSystem now also has direct mechanisms for integrating with PIC models — including support for importing optical S-parameters and the PIC Waveguide component, intended for modelling passive or electrically driven optical waveguides.

And where does Lambda Research's role begin?

Two worlds have to be clearly separated. OptiFDTD, OptiBPM, and OptiMode mainly describe problems related to the wave nature of light in photonic structures. OSLO and TracePro from Lambda Research are primarily tools of classical optical engineering based on ray analysis - for designing, analysing, and optimising optical and illumination systems. These are not replacements for one another.

OSLO : the classical optical system around the PIC

OSLO is a tool for designing optical systems, especially lens systems — it uses sequential ray tracing, analysis, and optimisation. It can be very useful when the PIC is part of a larger system: laser → lenses → PIC, or PIC → lenses → detector. If the problem is optimising the ray parameters of a lens system, there is no need to use FDTD for the whole system.

TracePro : when you need 3D and non-sequential ray tracing

TracePro deals mainly with ray optics and the analysis of complex optical systems. Lambda Research distinguishes the two tools clearly: OSLO - primarily lens-system design and sequential ray tracing; TracePro — complex 3D models and non-sequential ray tracing.

This matters for a whole photonic module. Suppose we have a PIC + housing + lens + fiber + mechanical elements. Inside the PIC we analyse wave propagation. Outside the PIC, different questions arise: where does the light go? What are the reflections and losses due to the geometry? How do the rays behave in space? Is there unwanted scattered light? How does the mechanical assembly affect the optical paths? Here TracePro — working with CAD models and complex 3D geometry — is a much more natural tool. We develop the topic of scattered light in the article What is stray light and why does it matter?.

Wave optics vs. ray optics

Wave optics — we describe light as an electromagnetic field; we are interested in amplitude, phase, interference, diffraction, polarisation, modes, and coupling. Typical tools: OptiMode, OptiBPM, OptiFDTD, OptiOmega.

Ray optics — we describe light with rays; we are interested in the geometry of the optical path, reflections, refraction, lenses, apertures, energy distribution, and stray light. Typical tools: OSLO, TracePro. Lambda Research emphasises the difference between sequential ray tracing in OSLO and non-sequential in TracePro. More on the methods → Methods for modelling optical phenomena.

Example: designing a Mach–Zehnder interferometer

Consider a simple MZI (Mach–Zehnder Interferometer) — two splitting/combining elements and two arms whose relative phase affects the interference result. The design can be approached in stages:

• Waveguide — we define the geometry and materials; we ask which modes the structure guides → OptiMode.
• Coupling element — we analyse the signal-splitting structure; we ask how the energy divides between the ports → OptiBPM, and where a more detailed analysis is needed → OptiFDTD / OptiOmega.
• Device characteristics — the result can be the element's port characteristics.
• Larger circuit — we assemble splitter + arm 1 + arm 2 + combiner; if the element is electrically driven → OptiSPICE.
• System — if the MZI is part of a transmitter or receiver → OptiSystem.

This is exactly what hierarchical PIC modelling is.

The core workflow

The whole process can be reduced to a single schema of modelling levels:

material + geometry → mode analysis (OptiMode) → propagation (OptiBPM) → detailed electromagnetic analysis (OptiFDTD / OptiOmega) → device parameters / compact model → PIC circuit + electronics (OptiSPICE) → optical system (OptiSystem) → external optics / module / housing (OSLO / TracePro).

This does not mean every project has to go through all these programs — it is a hierarchy of possible modelling levels, not a mandatory procedure.

Which program should you use?

Optiwave groups its tools by exactly these levels: from component and waveguide design, through optoelectronic-circuit simulation, to the system level. We set out the criteria for choosing a package in the guide How to choose optical simulation software?.

A PIC is not one model

This is the most important conclusion: a photonic integrated circuit should not be treated as one big electromagnetic model. It is a system that can be described at several levels:

• a single waveguide cross-section → a mode model;
• a photonic device → BPM or FDTD;
• an element represented parametrically → a compact model / port parameters;
• a PIC circuit → a circuit model;
• PIC + electronics → an optoelectronic simulation;
• a PIC as part of a transmission system → a system simulation;
• PIC + classical optics + housing → an opto-mechanical model and ray tracing.

That is why the question „which program is best for a PIC?” is poorly posed. A better question is: „which level of physics do I want to model right now?”. If the answer is modes — we start with OptiMode; if propagation in a waveguide — OptiBPM; if a detailed electromagnetic response of the device — OptiFDTD or OptiOmega; if the interaction of optics and electronics — OptiSPICE; if the whole optical link — OptiSystem. If the problem is already outside the chip itself — in classical optics, lenses, 3D geometry, or stray light — OSLO and TracePro can enter the workflow.

This hierarchy of models is one of the key elements of professional PIC design: maximum accuracy where it is needed, and an appropriately reduced model where the speed of simulating the whole circuit matters.