When telecoms lay cables, the biggest cost is digging the trench. The cable itself is cheap. So, they lay way more fiber than they need. This unused, unlit fiber is called "Dark Fiber".
1. Why Buy Dark Fiber?
Banks, Governments, and Big Tech (Google, Facebook) lease Dark Fiber to create their own private networks. They attach their own lasers to the ends.
- Security: The data never touches the public internet. It stays on a dedicated physical strand of glass.
- Speed: Light speed. No routers to slow it down.
- Control: You decide the protocols and the bandwidth.
2. DWDM
Dense Wavelength Division Multiplexing. This technology allows sending multiple colors of light down a single fiber strand. With DWDM, a single pair of fibers can carry Terrabits per second.