In the present paper, we demonstrate the first outdoor free-space optical communication (FSOC) system with real-time video transmission in a 2-μm-band with a state-of-the-art data rate performance of 10 Gbit/s and a communication distance of ∼177 m. The measured wide-open eye diagram and zero bit-error-rate also validated the high communication quality. The FSOC system is constructed with all homemade components, including a single-frequency (SF) thulium-holmium co-doped fiber laser (THDFL), a modulation module, a demodulation module, three thulium-doped fiber amplifiers, an optical bandpass filter, two optical-antennas, and a display system. As the most important component, the SF THDFL, enabled by a uniform fiber Bragg grating and a passive fiber ring-compound cavity, is self-designed and fabricated by our group, presenting high lasing stability centered at 2048.98 nm, an optical signal-to-noise ratio of >81 dB, a relative intensity noise of <−128.18 dB/Hz at >0.5 MHz and a linewidth of 5.290 kHz under a measurement integration time of 0.001 s. Our work is an important step forward for practical outdoor FSOC in a 2-μm-band, proving useful for engineers and scientists when our SF THDFL is implemented as a low-cost, high-performance laser source for the FSOC system.

Mode excitations in D-shaped multimode fiber are analyzed in detail in terms of the effect of launch offset, polishing depth, and length of the D-shaped section. Based on the analysis, specklegrams from D-shaped multimode fiber structures are pro-posed to be applied in refractive index sensing. Glycerol solutions with different concentrations were used to study the relationship between the zero-mean normalized cross-correlation coefficient (ZNCC) of fiber output specklegrams and the surrounding refrac-tive index. Responses of different D-shaped fiber structures are in-vestigated and compared. For all of the structures, regular gradual changes of the ZNCC for each specklegram with the refractive in-dex are found in a wide refractive index range of 1.352-1.444, which shows the feasibility of refractive index sensing. A high sen-sitivity being 10.47 RIU-1 is obtained by the structure with the pol-ishing depth being 18μm and length being 5cm. The proposed re-fractive sensing scheme is easy to realize with wide measurement range and high flexibility.

A novel approach for multimode fiber (MMF) curvature sensing based on the texture feature of the specklegram from the fiber facet is proposed, developed and demonstrated. Different from previously reported methods, the texture feature vector of the fiber specklegram is used as the descriptor of the curvature of the fiber. After the specklegrams from two kinds of MMF under different curvatures are recorded, the texture feature vector of each specklegram is first extracted using the uniform local binary pattern algorithm. Then, the texture feature vector is mapped into the target curvature in a nonlinear manner using a one-dimensional convolutional neural network (1D CNN). In the experiment, the model trained by the texture features of fiber specklegrams with a core diameter of 50 $\boldsymbol {\mu }\text{m}$ exhibits high prediction accuracy and good generalization ability in the curvature range of 1.55-6.93 $\text{m}^{-1}$ . In the first experiment, good recognition results are provided by the texture feature of the MMF specklegram, for which the average curvature recognition accuracy is 100%. In the second experiment, the prediction error for 91.4% of the samples in the test set is within a range of $\boldsymbol {\pm }{\mathbf {{0}.{1}\,\,m}}^{\mathbf {-{1}}}$ . In addition, this approach has significantly better prediction ability and robustness than the traditional approach, which processes the specklegram image directly using a two-dimensional convolutional neural network (2D CNN) for sensing. The experimental results demonstrate the fiber curvature sensing capability of the proposed method based on the analysis of texture feature using a 1D CNN.

A modified polarization-maintaining Sagnac loop mirror is proposed, and the simulated transmission spectrum of the proposed filter is in accordance with experimental transmission spectrum. Based on the proposed filter, a multi-wavelength thulium-doped fiber laser is constructed and experimentally demonstrated. The stable multi-wavelength oscillation within 60 minutes is achieved through the nonlinear polarization rotation effect enhanced by the highly nonlinear fiber. By fine tuning the polarization state, up to 35 lasing lines are obtained within a 10-dB bandwidth with wavelength separation of ~1.38 nm at room temperature. The wavelength shift and the output power fluctuation are less than 0.04 nm and 1.69 dB, respectively. In addition, by changing the optical coupling ratio in the Sagnac loop mirror, different outputs with 7, 8, 16 lasing lines are also obtained.