Stimulated Phonon Oscillations in a Multi-core Optical Fiber

Opto-mechanical oscillators that generate coherent acoustic waves are drawing much interest, in both fundamental research and applications. Narrowband oscillations can be obtained through the introduction of feedback to the acoustic wave, in so-called "phonon lasers". Most previous realizations of this concept relied on radiation pressure and moving boundary effects in micro- or nano-structured media. Other demonstrations in bulk crystals required cooling of the medium to cryogenic temperatures. In this work, stimulated emission of highly-coherent, guided acoustic waves is achieved based on inter-core, opto-mechanical cross-phase modulation in a commercially available multi-core optical fiber, at room temperature. The fiber is inserted within an opto-electronic cavity loop. Pump light in one core drives acoustic waves via electrostriction. In contrast to the optical field, the excited vibrations extend across the entire cross section of the fiber and are not confined to a single optical core.

An optical probe wave at a different physical core and a different wavelength undergoes photo-elastic phase modulation when propagating along the vibrating fiber. Coupling between probe and pump waves is exclusively mechanical: The cores of the fiber are optically isolated, with negligible (<-40dB) optical coupling. Furthermore, in order to assure that there is no direct optical feedback in the loop, the signal in the probe core is optically filtered. The probe’s phase modulation is translated to an intensity modulation via a Sagnac interferometer loop configuration. The probe wave is then detected and converted to the radio-frequency (RF) domain. The RF signal is amplified and fed back to modulate the intensity of the pump optical wave.

Stable oscillations can take place when the optical pump power and the RF gain are sufficient to compensate for losses in the conversion of the waveform from the optical to the mechanical domain. At the same condition, the stimulation of the acoustic waves within the cross-section of the fiber balances losses due to acoustic dissipation.

Single-frequency mechanical oscillations at hundreds of MHz frequencies are obtained, with side-mode suppression that is better than 55 dB. A sharp threshold and rapid collapse of the linewidth above threshold are observed. The linewidths of the acoustic waves oscillations are on the order of 100 Hz, and the relative variations in frequency are on the order of 0.1-1 ppm. The frequency of oscillations may be switched among several values by propagating the pump or probe waves in different cores and locking the acoustic vibrations to different acoustic modes of the fiber.