Birefringence-Induced Trains of High-Rate Pulses in a Mode-Locked Fiber Laser

The output of a mode-locked erbium-doped ring fiber laser incorporating a section of a polarization-maintaining (PM) fiber is investigated in both numerical simulations and experiments. With proper inline polarization control, the laser can be set to emit a train of pulses, separated by the differential group delay of the PM section. Repetition rates as high as 500 GHz are experimentally observed. The results provide an added insight into the role of birefringence in mode-locked lasers based on nonlinear polarization rotation

Electro-opto-mechanical radio-frequency oscillator driven by guided acoustic waves in standard single-mode fiber

An opto-electronic radio-frequency oscillator that is based on forward scattering by the guided acoustic modes of a standard single-mode optical fiber is proposed and demonstrated. An optical pump wave is used to stimulate narrowband, resonant guided acoustic modes, which introduce phase modulation to a co-propagating optical probe wave. The phase modulation is converted to an intensity signal at the output of a Sagnac interferometer loop. The intensity waveform is detected, amplified and driven back to modulate the optical pump. Oscillations are achieved at a frequency of 319 MHz, which matches the resonance of the acoustic mode that provides the largest phase modulation of the probe wave. Oscillations at the frequencies of competing acoustic modes are suppressed by at least 40 dB. The linewidth of the acoustic resonance is sufficiently narrow to provide oscillations at a single longitudinal mode of the hybrid cavity. Competing longitudinal modes are suppressed by at least 38 dB as well. Unlike other opto-electronic oscillators, no radio-frequency filtering is required within the hybrid cavity. The frequency of oscillations is entirely determined by the fiber opto-mechanics

Distributed opto-mechanical analysis of liquids outside standard fibers coated with polyimide

The analysis of surrounding media has been a long-standing challenge of optical fiber sensors. Measurements are difficult due to the confinement of light to the inner core of standard fibers. Over the last two years, new sensor concepts have enabled the analysis of liquids outside the cladding boundary, where light does not reach. Sensing is based on opto-mechanical, forward stimulated Brillouin scattering interactions between guided light and sound waves. In most previous works, however, the protective polymer coating of the fiber had to be removed first. In this work, we report the opto-mechanical analysis of liquids outside commercially available, standard single-mode fibers with polyimide coating. The polyimide layer provides mechanical protection but can also transmit acoustic waves from the fiber cladding towards outside media. Comprehensive analysis of opto-mechanical coupling in coated fibers that are immersed in liquid is provided. The model shows that forward stimulated Brillouin scattering spectra in coated fibers are more complex than those of bare fibers, and strongly depend on the exact coating diameter and the choice of acoustic mode. Nevertheless, sensing outside coated fibers is demonstrated experimentally. Integrated measurements over 100 meters of fiber clearly distinguish between air, ethanol and water outside polyimide coating. Measured spectra are in close quantitative agreement with the analytic predictions. Further, distributed opto-mechanical time-domain reflectometry mapping of water and ethanol outside coated fiber is reported, with a spatial resolution of 100 meters. The results represent a large step towards practical opto-mechanical fiber sensors

Vector analysis of stimulated Brillouin scattering amplification in standard single-mode fibers

The polarization properties of stimulated Brillouin scattering (SBS) amplification or attenuation in standard single-mode fibers are examined through vectorial analysis, simulation and experiment. Vector propagation equations for the signal wave, incorporating SBS and birefringence, are derived and analyzed in both the Jones and Stokes spaces. The analysis shows that in the undepleted pump regime, the fiber may be regarded as a polarization-dependent gain (or loss) medium, having two orthogonal input SOPs, and corresponding two orthogonal output SOPs, for the signal, which, respectively, provide the signal with maximum and minimum SBS amplification (or attenuation). Under high Brillouin gain conditions and excluding zero-probability cases, the output SOP of arbitrarily polarized input signals, would tend to converge towards that of maximum SBS gain. In the case of high SBS attenuation the output SOP of an arbitrarily polarized signal would approach the output SOP corresponding to minimum attenuation. It is found that for a wide range of practical pump powers (<= 100 mW) and for sufficiently long fibers with typical SBS and birefringence parameters, the signal aligned for maximum SBS interaction will enter/emerge from the fiber with its electric field closely tracing the same ellipse in space as that of the pump at the corresponding side of the fiber, albeit with the opposite sense of rotation. The analytic predictions are experimentally demonstrated for both Stokes (amplification) and anti-Stokes (attenuation) signals

Polarization-induced distortion in stimulated Brillouin scattering slow-light systems

The vector analysis of stimulated Brillouin scattering amplification in birefringent fibers is extended to include signal pulses. The analysis finds that the different slow-light delays experienced by the states of polarization corresponding to maximum and minimum gain may result in severe pulse distortion. Thus, a generally polarized pulse, experiencing only a moderate gain, can become broader than a pulse aligned for maximum gain and delay. The effect is demonstrated in both numerical simulations and experiments