We present a theoretical analysis and an experimental study of the impacts of external optical feedback on dual-frequency fiber lasers. The external optical feedback can effectively suppress the phase noise of the beat notes of dual-frequency fiber lasers, provided some requirements are satisfied. The polarization of the optical feedback is important for the fiber laser’s stability, and it can also tune the beat note frequency. A side effect of external optical feedback, as demonstrated in the experiments, is lowering the sensitivity of the dual-frequency fiber laser-based sensors, although such degradation is not obvious.

We demonstrate an ultralow-noise single-photon detection system based on a sensitive photomultiplier tube (PMT) with precise temperature control, which can capture fast single photons with intervals around 10 ns. By improvement of the electromagnetic shielding and introduction of the self-differencing method, the dark counts (DCs) are cut down to ～1%. We further develop an ultra-stable PMT cooling subsystem and observe that the DC goes down by a factor of 3.9 each time the temperature drops 10°C. At 20°C it is reduced 400 times with respect to the room temperature (25°C), that is, it becomes only 2 counts per second, which is on par with the superconducting nanowire detectors. Meanwhile, despite a 50% loss, the detection efficiency is still 13%. Our detector is available for ultra-precise single-photon detection in environments with strong electromagnetic disturbances.

Polarization-based optical communications are attracting more attention recently, where the crucial points are polarization features and their measurements. Based on the Müller matrix method, we obtain measurable expressions for the polarization-dependent gain (PDG) and the loss of polarization orthogonality (LPO), while give the boundary of the LPO for any PDG devices. We experimentally demonstrate that non-linear LPO can be created in a semiconductor optical amplifier and find that the LPO will slightly skim over the boundary near the threshold of the injected current. Furthermore, an empirical formula is achieved to gauge the LPO-induced power penalty, which is proven to be valid in differential polarization shift-keying transmission by executing a bit error rate measurement. Our conclusions are applicable to non-orthogonal polarization cases and valuable to polarization-related communications, even orbital angular momentum multiplexing.