Single photon detector (SPD) has a maximum count rate due to its dead time, which results in that the dynamic range of photon counting optical time-domain reflectometry (PC-OTDR) de-creases with the length of monitored fiber. To further improve the dynamic range of PC-OTDR, we propose and demonstrate an externally time-gated scheme. The externally time-gated scheme is realized by using a high-speed optical switch, i.e. a Mach-Zehnder interferometer, to modulate the back-propagation optical signal, and to allow that only a certain segment of the fiber under test is monitored by the SPD. The feasibility of proposed scheme is first examined with theoretical analysis and simulation; then we experimentally demonstrate it with our experimental PC-OTDR testbed operating at 850 nm wavelength. In our studies, a dynamic range of 30.0 dB is achieved in a 70 meters long PC-OTDR system with 50 ns ex-ternal gates, corresponding to an improvement of 11.0 dB in dynamic range comparing with no gating operation. Further-more, with the improved dynamic range, a successful identifica-tion of a 0.37 dB loss event is detected with 30-seconds accu-mulation, which could not be identified without gating operation. Our scheme paves an avenue for developing PC-OTDR systems with high dynamic range.

Optical phase locking is a critical technique in space coherent optical communication and active coherent laser beam combining. In a typical optical phase locking loop based on nonplanar ring oscillators, the pull-in range is normally less than 1 MHz, limited by loop delay and frequency tuning bandwidth of the laser source. Phase locking cannot be achieved at large initial frequency differences. In this work, a fast laser frequency acquisition method is demonstrated. The frequency difference between the signal and local lasers was measured via frequency dividing and period counting, and the frequency control signal was generated by a frequency discrimination and control module, to reduce the frequency difference to the pull-in range of the loop. Under the coordinating function of the loop filter and the frequency discrimination and control module, phase locking under a large initial frequency difference was achieved. The frequency acquisition range reached 164 MHz, and the acquisition and locking time was measured to be 440 ms. Additionally, the acquisition time was shortened with the decrease in initial frequency difference.

We present a full quantum mechanical model for a composite-ring erbium-doped fiber laser with a feedback loop, with a view to predict the achievable intensity noise reduction. By adding an electronic feedback term to the noise transfer function of the free running composite-ring fiber laser, an analytical expression is developed for the intensity noise spectrum of the current system. For ring fiber lasers, the model shows that the subcavity can suppress the noise at the high frequency and reduce the cutoff frequency to the quantum noise limit, and the feedback loop has a significant impact on suppressing the intensity noise near the relaxation oscillation.

We present a high-precision optical phase-locking based on wideband acousto-optical frequency shifting. Increasing the modulating bandwidth stabilizes the loop at a high loop gain, thus improving phase correction capability. An optical phase-locked loop with a wide control bandwidth is constructed. The closed-loop residual phase error is only 0.26o or smaller than \lambda/1000. The loop exhibits excellent correction capability for high-frequency noises. The correctable frequency range reaches 35 kHz when the noise amplitude is +(-)\lambda/2, and becomes even wider for smaller noise amplitudes.

A novel multi-beam photonic receiver with a variable beam count has been proposed. The reconfigurable characteristic of the system is enabled with the variable splitters and optical switch arrays. According to the number of beams it is due to receive, the splitter with suitable ports is chosen to split by controlling the optical switch in order to configure the optical path. After splitting, the multi-wavelength optical carriers are sent to the wavelength division multiplexer to combine and complete the multi-beamforming process. Two structures have been designed to realize the reconfigurable function, while the optimized structure is proved by using several benchmarks for comparison. The operation step of the reconfigurable multi-beam receiver is then given. The first simulations validate that the proposed architecture can dynamically receive the multi-beam with a variable number by reconfiguring the state of the optical switch. The second simulation of the influence on the received beam pattern was performed under a different amplitude and phase inconsistency random values. The results show that the amplitude and phase inconsistency will decline the quality of the multi-beam. When the amplitude and phase inconsistency are within limits, the difference in the optical path within the outputs must be balanced and compensated during the proposed system’s fabrication and deployment.