Components & building blocks
The standard library of photonic devices you compose into circuits — what each one does, when to reach for it, and how Qfactr makes them behave like the real parts.
A photonic integrated circuit is assembled from a relatively small vocabulary of building blocks. Each block manipulates light in a specific way — guiding it, splitting it, combining it, delaying it, filtering it, or converting it to and from the electrical domain. Larger functions like filters, switches, and transceivers are built by wiring these primitives together with waveguides.
This page is a working catalog of the standard components you will find in most photonic libraries. The physics is general PIC knowledge; the final section describes how these parts are represented inside Qfactr.
Passive components
Passive components shape and route light without an external control signal. They are the structural backbone of almost every circuit.
Waveguides
Straight and bend waveguides are the wires of photonics: they confine and carry an optical mode from one point to another. Unlike electrical wires, geometry has a direct optical cost — bends below the minimum radius radiate light away, and every micrometer of path adds propagation loss. See Waveguides & routing for the loss mechanisms in detail.
Splitters and couplers
Several devices split one optical signal into two (or combine two into one), differing in how the split ratio is set and how broadband they are:
- Directional coupler — two waveguides brought close enough that light evanescently couples between them. The coupling (split) ratio depends on the gap and interaction length, and varies with wavelength. Used for tunable power taps and as the coupling element in interferometers and resonators.
- MMI coupler — a multimode interference device: light entering a wide multimode section self-images at the output ports, giving a fixed, fabrication-tolerant split (for example 1×2 or 2×2). Used where you want a robust, broadband, well-defined split ratio.
- Y-splitter — a Y-shaped junction that divides input power evenly between two output branches (and combines in reverse). Compact and broadband; used for simple 50/50 power division such as feeding the two arms of an interferometer.
Resonators and interferometers
- Ring / microring resonator — a looped waveguide coupled to one or more bus waveguides. It resonates at wavelengths whose round trip is a whole number of wavelengths, making it a compact, wavelength-selective filter, modulator core, or sensor. The free spectral range is set by the ring radius.
- Mach–Zehnder interferometer (MZI) — splits light into two arms and recombines it, so the output depends on the phase difference between the arms. The workhorse for switches, modulators, and filters; placing a phase shifter on one arm makes the response controllable.
Active and I/O components
Active components change the light in response to an electrical control signal; input/output components move light between the chip and the outside world; detectors return light to the electrical domain.
- Phase shifter — applies a controllable phase delay to the light in a waveguide. Thermo-optic shifters use a heater to change the index via temperature (compact, low-loss, slower); electro-optic shifters use an applied field (faster, used for high-speed modulation). The control element behind tunable MZIs and rings.
- Modulator — encodes an electrical signal onto an optical carrier by modulating its intensity or phase, typically built from an MZI or ring driven by an electro-optic phase shifter. The transmit side of an optical link.
- Photodetector — absorbs light and produces a photocurrent, converting the optical signal back to electrical. The receive side of a link, and the readout for sensing.
- Grating coupler — a periodic structure that diffracts light between an on-chip waveguide and a fiber held above the chip. Allows wafer-scale testing and flexible fiber placement; inherently wavelength- and polarization-dependent.
- Edge coupler — couples light at the cleaved or polished chip facet to a butt-coupled fiber. Typically broadband and lower-loss than grating couplers, at the cost of needing facet access and careful alignment.
Quick reference
| Component | Function | Typical use |
|---|---|---|
| Straight / bend waveguide | Confines and transports an optical mode; bends redirect it | Routing light between components |
| Directional coupler | Evanescently couples light between adjacent waveguides | Tunable power taps; coupling into rings and MZIs |
| MMI coupler | Self-imaging split with a fixed, fabrication-tolerant ratio | Robust broadband 1×2 / 2×2 splitting and combining |
| Y-splitter | Splits power evenly across two branches | Simple 50/50 division, e.g. feeding MZI arms |
| Ring / microring resonator | Wavelength-selective resonance via a looped waveguide | Filters, modulator cores, sensors, WDM (de)multiplexing |
| Mach–Zehnder interferometer | Phase-difference-dependent interference between two arms | Switches, modulators, tunable filters |
| Phase shifter (thermo-/electro-optic) | Applies a controllable phase delay | Tuning MZIs and rings; driving modulators |
| Modulator | Encodes an electrical signal onto an optical carrier | Transmit side of an optical link |
| Photodetector | Converts light to photocurrent | Receive side of a link; sensing readout |
| Grating coupler | Diffracts light between waveguide and fiber from above | Wafer-scale test; flexible fiber I/O |
| Edge coupler | Butt-couples light at the chip facet | Low-loss, broadband fiber I/O |
Components in Qfactr
In Qfactr these are not abstract symbols. Components come from PDK-backed libraries: each part carries its physical footprint, pin positions in real micrometers, and S-matrix (S-parameter) data describing how it transforms light. Because that information travels with the part, what you place on the canvas behaves like the device it represents rather than a placeholder on a schematic.
That backing is what lets the rest of the workspace stay grounded. Routes connect real pins, transmission and loss are derived from the actual geometry, and the connection state of every pin is visible at a glance — pins render red when unconnected and green once a waveguide reaches them. You browse and place parts from the searchable library against the active PDK; the starter library is the Demo-PDK.