Abstract
In this paper, we study the implementation of a secure key distribution system based on an ultra-long fiber laser with a bi-directional erbium-doped fiber amplifier. The resilience of the system was tested against passive attacks from an eavesdropper. A similarity was observed in the spectra for both secure configurations of the system and no signature that would allow an eavesdropper to obtain the secure state of the system was observed during the state transitions.

Abstract
In this paper, a different Fiber Loop Mirror (FLM) configuration with two circulators is presented. This configuration is demonstrated and characterized for sensing applications. This new design concept was used for strain and torsion discrimination. For strain measurement, the interference fringe displacement has a sensitivity of (0.576 +/- 0.009) pm.mu epsilon(-1). When the FFT (Fast Fourier Transformer) is calculated and the frequency shift and signal amplitude are monitored, the sensitivities are (-2.1 +/- 0.3) x 10(-4) nm(-1) mu epsilon(-1) and (4.9 +/- 0.3) x 10(-7) mu epsilon(-1), respectively. For the characterization in torsion, an FFT peaks variation of (-2.177 +/- 0.002) x 10(-12) nm(-1)/degrees and an amplitude variation of (1.02 +/- 0.06) x 10(-3)/degrees are achieved. This configuration allows the use of a wide range of fiber lengths and with different refractive indices for controlling the free spectral range (FSR) and achieving refractive index differences, i.e., birefringence, higher than 10(-2), which is essential for the development of high sensitivity physical parameter sensors, such as operating on the Vernier effect. Furthermore, this FLM configuration allows the system to be balanced, which is not possible with traditional FLMs.

Abstract
In this letter, a new configuration based on two fiber loop mirrors in series using a 3x3 coupler is demonstrated. This configuration is compact and can explore the Vernier effect. The results obtained were a sensitivity of 74.9 +/- 0.2 pm/ mu epsilon for the Vernier envelope and 6.41 +/- 0.01 pm/mu epsilon for the Vernier carrier. These results were compared to the sensitivity of the setup using only one fiber loop mirror interferometer, obtaining 14.41 +/- 0.01 pm/mu epsilon. This means a magnification factor of 5.2 for the Vernier envelope. Theoretical treatment of the system was carried out, using Jones matrix calculus, and the subsequent simulation results are also presented, having obtained good agreement with the experimental data.