Wavelength multiplexing vs parallel optics: trading fiber count against wavelengths

Engineers scaling bandwidth between data-center racks can either add parallel fibers with one wavelength each, or send several wavelengths over the same fiber. Wavelength-division multiplexing keeps fiber count flat but requires costly multiplexing and demultiplexing optics, tighter laser control, and risks losing every channel to a single shared-path fault. Parallel single-wavelength optics shifts spending into operating costs and simpler hardware but needs more glass and devices. The choice also depends on budgeting, since capital and operational spending are approved differently, and on which part of the link is actually the bottleneck. Distance is the deciding factor: over short runs within a rack or row, extra fiber is cheap and multiplexing adds overhead, while over campus or long-haul distances packing wavelengths onto existing fiber is much cheaper.