270-km Ultralong Raman Fiber Laser

We analyze the physical mechanisms limiting optical fiber resonator length and report on the longest ever laser cavity, reaching 270 km, which shows a clearly resolvable mode structure with a width of $\sim 120\mathrm{Hz}$ and peak separation of $\sim 380\mathrm{Hz}$ in the radio-frequency spectrum.

Figure 1

Schematic of the ultralong Raman fiber laser design and calculated optical powers distribution.

Figure 2

Experimental demonstration of the resolvable mode structure with the spectral spacing following the classical laser law $\Delta \nu =c/2nL$. Insets: intensity beating spectra for 105, 208, and 270 km lasers.

Figure 3

Mode beating peak width (FWHM) for 6.6., 22, 44, 84, 105, 165, 208, and 270 km lasers (differently marked points) and their linear fit. Corresponding mode spacing values are shown by horizontal lines.