External Optical Feedback Induced Noise in DFB Fiber Laser Sensor System

External optical feedback effects due to reflection, Rayleigh backscattering and coherent Rayleigh backseattering in fiber distributed feedback （DFB） fiber laser sensor system have been investigated. If the feedback intensity exceeds critical amount, excess noise would be induced in the demodulator. The maximum tolerable intensity back-reflection coefficient Rc and backscattering coefficient Sc into a fiber DFB laser with lead fiber length from I m to 37.5 km before the onset of instabilities are shown. Rc is found to decrease with increasing lead fiber length while Sc was relatively invariable with varying fiber length. The coherent Rayleigh backscattering （CRBS） would induce neglectable noise with a lead fiber exceeding 13.5 kin. To eliminate these noises, one or two isolators should be incorporated in the system.

Cavity-Mode Properties of Semiconductor Lasers Operating in Strong-Feedback Regime

This paper investigates the modal properties of semiconductor lasers operating in the strong-feedback regime. Analytical expressions are developed based on an iterative travelling-wave model, which enable a complete and quantitative description of a compound cavity mode in its steady state. Additional information is provided about the physical inside into a compound laser system, such as a bifurcation diagram of the compound cavity modes for full variation range （from 0 to 1） of the external reflection coefficient and a more general shape for the diagram of photon density versus mode phase - this latter will reduce to the classical ＂ellipse＂ in the weak-feedback regime. It is shown that in the strong-feedback regime, a feedback laser is characterized by a small mode number and a high density of photons. This behavior confirms previous experimental observations, showing that beyond the coherence-collapse regime, the compound laser system could be re-stabilized, and that as a result power-enhanced low-noise stable laser operation with quasi-uniform pulsation is possible with external-mirror reflectivity close to 1. Moreover, it is also shown that for a compound system operating in the strong-feedback regime, an anti-reflection treatment of a laser can significantly reduce its current threshold, and that in the absence of this treatment excitation of a minimum-linewidth mode with higher output power would be possible inside such a system. Finally, it is shown that in the weak-feedback regime except for a phase shift the iterative travelling-wave model will reduce to the Lang-Kobayashi model in cases where the product of the feedback rate and the internal round-trip time is much less than unity （that would mean in situations of as-cleaved lasers）.