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We propose and experimentally demonstrate a centralized PON monitoring scheme using the reflection spectrum of a single fiber Bragg grating. Each fiber Bragg grating used in the scheme adopts different central reflection wavelength or 3-dB reflection bandwidth. The unique reflection spectrum is generated after each fiber Bragg grating used is interrogated by a broadband probe source. These generated reflection spectra are used to identify and distinguish the drop fiber links. The similarity analysis model is established to solve the problem of waveform superposition identification for different 3-dB reflection bandwidths with the same central reflection wavelength, which can further improve the network monitoring capacity. The proposed scheme provides a simple and effective monitoring solution for large-capacity network, especially for high-density network with small end-user distance difference.

Brillouin optical time-domain analysis (BOTDA) that operates over a long sensing fiber is prone to be affected by the detrimental non-local effects (NLE); since NLE can distort Brillouin gain spectrum (BGS), therefore correctly retrieving Brillouin frequency shift (BFS) is very challenging. Recently, the basic artificial neural networks (B-ANN) has been demonstrated to retrieve BFS effectively, in the case that a distortion-free BGS can be obtained. However, in the more general cases with NLE, a neural network for retrieving BFS in BOTDA data has not been proposed. In this paper, firstly the physical origin of NLE is analyzed theoretically and experimentally in detail. Then a specific ANN (NLE-ANN) is proposed to deal with the BOTDA data affected by NLE for the first time. The experimental verifications show that with the cooperative implementation of NLE-ANN and B-ANN, the effective retrieval of BFS along the whole fiber can be achieved, even though the BGS has been affected by NLE. This work proposes a promising approach in traditional BOTDA, because the upper limit of probe power could be raised by artificial intelligence to boost the sensing performance.

We proposed and demonstrated a new structure, which employs a single laser in Brillouin optical time-domain analysis sensor assisted by distributed Brillouin amplification. This structure can avoid the stability problem caused by laser frequency drift. In addition, we used an optical comb technique to suppress the distortion of pump pulse. Moreover, the feasibility of an optical pulse coding technology in this system was studied. In particular, we explored the influence of coding parameters on the transient gain saturation of the coded pulse. Results show that the coded pulse space and coding length must be optimized to suppress its transient gain saturation, by which the sensing with lower decoding error can be achieved.

In this paper, we make a comprehensive study on a highly efficient half-open cavity design for high power random fiber laser (RFL). With the theoretical analysis, we optimize the cavity's fiber length for getting higher output power within the scheme, i.e., shorter fiber length is preferred for efficiently harvesting the first order random lasing at the open end of the cavity. As the verification of the theory, we experimentally demonstrate a high output power (7 W), highly efficient (70% optical conversion efficiency) RFL working at 1140 nm, using 10 W 1090 nm laser as the pump source and only 1 km standard single-mode fiber as the distributed cavity.

Random distributed feedback fiber laser (RDF-FL) based on combination of Er-doped fiber (EDF) and single-mode fiber (SMF) is proposed in this paper. Through pumping of both the EDF (i.e., 1480 nm pump) and the SMF (i.e., 1455 nm pump), random lasing is obtained. With increase of pump powers, different transitions between chaotic and stable status of the output spectrum are observed. Especially, single-peak random lasing can be obtained under the stable operation regime.

Random lasing (RL) characteristics of second-order random fiber laser formed solo by dispersion compensated fiber are studied. It is found that the threshold of first-order RL is only 0.45 W. In addition, a special route to stable second-order lasing is revealed, i.e., a special arc-shape output spectrum of second-order RL and three chaotic regimes during evolution from the first RL to the second-order one with increased pump power.

The Raman amplification can be employed to obtain the enhanced sensing performance in long-distance Brillouin optical time-domain analyzer (BOTDA) with higher temperature measurement accuracy over the whole sensing fiber. Theoretical investigation on the detected probe waveshape has been performed by using the probe-pump coupled equations combined with Raman amplification. The experimental characterization has been demonstrated to validate the theoretical prediction. The results show that, the 75 Km sensing distance can be achieved by utilizing the bi-directional Raman amplification with measurement accuracy of ±0.6°C over the whole sensing fiber. The larger Raman power and Brillouin pump power are helpful to improve the sensing signal-noise ratio (SNR). There exists an optimum range for the input probe power and the extinction ratio (ER) of input Brillouin pump pulse. The BOTDA with Raman amplification has considerable potentials to realize long- distance strain and temperature measurement for structural health monitoring and fire alarming, etc.

The passive mode-locked semiconductor laser (PMLL) can realize terahertz (THz) four-wave mixing (FWM) with high conversion efficiency and no need for pump input. Theoretical investigation on its FWM characteristics has been carried out by using the traveling wave and carrier rate equations combined with nonlinear dynamics of carrier density pulsation (CDP), carrier heating (CH), and spectral-hole burning (SHB). The stable mode-locked conjugate pulse with short-duration high-repetition frequency can be obtained by the PMLL. The pedestal-free conjugate pulse has narrower width and larger amplitude jitter compared with the pump wave. Smaller absorption recovery time is helpful to extend the stable conjugate output regime. There exists a down-limit for absorption recovery time to avoid self-pulsation (SP) instability. The larger injection current is required for smaller absorption recovery time to reach the given conversion efficiency. The larger conversion efficiency and narrower conjugate pulse can be obtained by proper design of distributed Bragg reflector (DBR) coupling strength. The DBR bandwidth must be designed to be within signal-pump detuning to avoid resonance-induced conjugate pulse distortion. The FWM in PMLL has considerable potential such as demultiplexing in optical time-division multiplexing systems.

By using coupled mode equations, the characteristic of terahertz wavelength converter based on four-wave mixing in lambda/4-shifted distributed feedback lasers is analyzed. The numerical simulation indicates that, the larger biasing current and smaller detuning can expand the dynamic range of conversion efficiency and extinction ratio, but worsen the frequency chirp. Due to the lasing effect, the peak modulation frequency of extinction ratio and frequency chirp occurs at relaxation oscillation frequency. The good conversion characteristic can be obtained by selecting system parameters reasonably. This configuration could have potentials in optical communications because of wideband, high-speed, and no need for pump input

Theoretical investigations on commanding the bistability and self-pulsation characteristics of two-segment bistable semiconductor laser diode (TBLD) using delayed optoelectronic feedback have been carried out in this paper. The effects of feedback gain and delayed time have been simulated numerically based on the rate equations. The results show that the stable region of TBLD varies periodically with delayed time, the bistable region shrinks with the increase of delayed time, stability can be enhanced for certain values of delayed time, and instability can be reached easily for negative delayed optoelectronic feedback. These conclusions establish the theoretical foundation to optimize and control the TBLD and may have potential applications in bistable switching and self-pulsation generation.

Starting from the coupled-mode equation and carrier rate equation, this paper has analyzed numerically the bistable dynamic characteristics of phased-shifted and chirped distributed-feedback semiconductor optical amplifiers (DFB-SOAs). The results show that phase shift first worsens the shape distortion of the falling edge of the output pulse and then alleviates it with the decreased initial detuning. This tendency can be suppressed by introducing chirp. We also proposed a novel configuration based on linearly variable current injection, which not only enhances the on-off switching ratio, but also has a similar transmission spectrum with phase-shift effects and similar bistable features, such as the wide frequency band and the flat upper branch of the hysteresis, with chirp effects.

The combination of Raman amplification and optical pulse coding (OPC) is proposed to reduce the frequency uncertainty in a long-distance Brillouin optical time-domain analyzer (BOTDA) under higher spatial resolution. The experimental characterization of temperature sensing with measurement accuracy ~ 1°C under ~ 2.5-m spatial resolution over a whole length of sensing fiber is demonstrated. The sensing fiber is composed of 49.6-km fiber at 10.8-GHz Brillouin frequency shift (BFS) and 25-km fiber at 11.005 GHz.