Optical communications stitches together every theme of the curriculum.
From Chapter 0 we inherit photons (so light is particles when it interacts with a photodiode), bandgap physics (so III-V semiconductors emit at the wavelengths they do), and the laser physics of stimulated emission (so coherent narrow-line sources are even possible).
From Chapter 9 we inherit Maxwell's equations (so guided modes have analytic solutions), Snell's law and total internal reflection (so the core traps light at all), and Fresnel coefficients (so we know how much light couples in at the air-fiber interface).
From Chapter 12 we inherit on-off keying, PAM-4, and QPSK to 64-QAM (the modulation formats we put on the carrier), error correction (which sets the OSNR threshold for reliable decoding), and the matched-filter receiver concept (which is what the coherent DSP implements).
From the upcoming chapters on hardware security we will inherit the realization that all this elegance, all this beautifully engineered glass and silicon, is also an attack surface, and that defense requires understanding the physics as well as the attacker does.
A trans-Pacific email goes through:
- A laser modulated by a Mach-Zehnder modulator at the data center on the West Coast.
- A coherent transponder running 64-QAM at 95 Gbaud, 800 Gbps per channel.
- An AWG combining 96 such channels onto one fiber.
- A submarine cable buried in the Pacific seabed.
- An EDFA every 80 km, pumped by a 980 nm laser through a copper conductor, for over 8000 km.
- An AWG demultiplexing the 96 channels at the receive landing station.
- A coherent receiver with a polarization-diverse 90-degree hybrid and four 100 GS/s ADCs.
- A DSP undoing 8000 km of accumulated chromatic dispersion in firmware.
- A series of routers, eventually a fiber-to-the-home XGS-PON link to the recipient's house.
- An ONT in their garage, a Wi-Fi link, a TCP/IP stack, an email client.
The whole journey takes about 100 ms and costs essentially nothing. Behind every line of that journey is a physical principle we have visited in this curriculum. The internet is a stack of physics, all the way down.