Tangentially related: I would like to wear a thin sensor band under my existing cheapo watch. I don't need a full on Bluetooth enabled smart watch most of the time (and hate charging it daily).
The thinner this tech gets, the closer we get to collecting metrics in this type of way.
In photonic integrated circuits, laser light enters some input port(s) and then travels through optical “wires” called waveguides that are arranged in ways that manipulate the light before it is finally detected using photodiodes.
In this paper, the main idea can be understood from Fig. 1a. A single pump laser with frequency νp enters a straight waveguide, but it is also coupled into three small loops called microring resonators, and there is an output port at the bottom right.
These rings are resonators because certain wavelengths of light can circulate around the loop constructively. In other words, after going around the ring once, the wave comes back in phase with itself and reinforces itself. The rings also have tiny heaters (resistive electrical wires) attached to them, allowing their resonances to be tuned slightly by changing the temperature.
What they show is that, using only one pump laser and careful tuning of the rings, the system can generate additional frequencies besides the original pump frequency.
This relies on nonlinear optics. In ordinary linear optics, the material response follows the electric field linearly, so light mostly stays at the same frequency. But in nonlinear materials, the response depends on the field strength itself, allowing the pump light to generate additional frequencies (“colors”) through nonlinear interactions.
A lot of the terminology in the paper like Kerr effects, χ(3) nonlinearity, four-wave mixing, parametric gain, idler frequencies, and cross-phase modulation, refers to different aspects of these nonlinear interactions.
The end result is that this optical component can generate multiple sets frequencies from a single input laser using these coupled nonlinear resonators.
I understood some of those words. I searched and read up a bit, and I now understand even less than I thought.
Seems related to this post, although OP source is much more detailed.