Highly coherent frequency-entangled photons based on parametric instability in active fiber ring cavity

Highly coherent frequency-entangled photons at telecom band are critical in quantum information protocols and quantum tele-communication. Photon pairs generated by spontaneous parametric down-conversion in nonlinear crystals or modulation instability in optical fibers exhibit random fluctuations. Here, we demonstrate highly stable frequency-entangled photons based on parametric instability in an active fiber ring cavity, where periodic modulation of dispersion excites parametric resonance, and the characteristic wave number is selected by the periodic modulation of resonator. Background-free autocorrelation of single-shot spectra reveals that spectra of parametric instability sidebands possess high coherence. The quantum properties are tested by the Hanbury Brown-Twiss measurement and Hong-Ou-Mandel interference. We conform the frequency-entanglement of two parametric instability sidebands by a spatial quantum beating with a fringe visibility of 97.9%. Our results prove that the parametric instability in active fiber cavity is effective to generate highly coherent frequency-entangled photon pairs, which would facilitate subsequent quantum applications.

A Hollow-Core Photonic-Crystal Fiber-Optic Gyroscope Based on a Parallel Double-Ring Resonator

A novel system structure of resonant fiber optical gyroscope using a parallel double hollow-core photonic crystal fiber ring resonator is proposed, which employs the double closed loop and reciprocal modulation–demodulation technique to solve the problem of the length mismatch between rings. This structure can suppress the residual amplitude modulation noise and laser frequency noise, essentially eliminating the influence of the Rayleigh backscattering noise and dramatically reduce the Kerr-effect-induced drift by three orders of magnitude. Thanks to its excellent noise suppression effect, the sensitivity of this novel system can approach the shot-noise-limited theoretical value of 8.94 × 10−7 rad/s assuming the length of the fiber ring resonator is 10 m.

Saturable absorption and its consequent effects in bistable erbium‐doped fiber ring laser

The linear absorption in erbium-doped fiber contributes to its excellent role in erbium-doped fiber amplifiers and lasers. A nonlinear optical contribution in the absorption of erbium-doped fiber is responsible for optical bistable action when it is present in a laser cavity. To quantify this effect, the variation of absorption coefficient is measured at different signal powers at multiple wavelengths in the C-band for different EDF lengths, and saturable absorption parameters such as the saturation power are extracted. Then the modification in the output characteristics of erbium-doped fiber ring laser with change in fiber length and in the presence of self-induced saturable absorption effect within the gain medium which leads to optical bistability is measured. By comparing the measured parameters obtained from saturable absorption in erbium-doped fiber and optical bistability in erbium-doped fiber ring laser, we estimated the length of the gain medium which acts as the saturable absorber inside the cavity of the laser. This is useful in constructing bistable lasers with optimized conditions. The temporal evolution of cavity loss and gain with the intra-cavity power and up- and down-thresholds helps in understanding why the down-threshold will be lesser than the up-threshold in bistable laser systems.