Vibration-induced PM noise measurements of a rigid optical fiber spool

The opto-electronic oscillator (OEO) has emerged in recent years as an excellent low-noise source that rivals the best RF oscillators over broad offset frequencies. The main sources of noise in an OEO are the laser and RF modulator, photo detector, loop amplifier, and the long fiber that is needed for high Q. Recent studies have shown that even by using state-of-the-art components and a low-loss long fiber, the phase-modulated (PM) noise of these OEOs fails to meet the theoretical value at offset frequencies close to carrier, from a few hertz to 1 kHz. The main cause for this shortfall is vibration effects on the optical fiber. External, environmental vibration causes mechanical distortions in the fiber that induce time-delay (phase) fluctuations. The spool onto which the fiber is wound is primarily responsible for imparting these vibration-induced delay fluctuations to the fiber and thus diminishing the performance of the OEO. In this paper, we compare the vibration-induced phase fluctuations of a 3 km optical fiber wound on spools made of four materials-metal, ceramic, plastic, and foam-covered plastic. We investigate fiber-on-spool winding and mounting techniques that reduce vibration susceptibility. We present residual PM measurements that compare the vibration sensitivity of an optical fiber wound on these different materials.