Laser frequency-fluctuation measurement employing an optical-fiber circulation loop

An optical circulation loop acting as a long fiber delay-line is used in a delayed-self-heterodyne setup for laser frequency-fluctuation measurement. Two-sample variance estimation at the averaging time up to 1 ms (corresponding to an equivalent delay-fiber length of 810 km) is demonstrated with the loop, which includes an acoustooptic modulator, 27-km delay fiber, and an erbium-doped fiber amplifier. A signal-to-noise-ratio (S/N) analysis shows that the amplified spontaneous emission (ASE) that accumulates in the loop degrades the S/N by the factor of 1/j/sup 2/, where j denotes the loop round-trip number.<>