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 switchable and tunable six-wavelength single-longitudinal mode (SLM) thulium-doped fiber laser in the 2050 nm band is proposed and experimentally demonstrated. Two cascaded sampled fiber Bragg gratings (SFBGs), acted as an integrated six-channel reflector, are to define the lasing wavelength channels in a multi-wavelength switchable fiber laser for the first time. The switchability of the fiber laser was realized by polarization-dependent loss modulation. SLM lasing in each channel was guaranteed by a novel passive dual-ring compound cavity (DRCC), composed of two symmetric 3 × 3 optical couplers (OCs). Six single-wavelength operations are easily obtained and switched, with the high optical signal-to-noise ratios of >71 dB, >72 dB, >62 dB for λ ₁₁ – λ ₁₃ and > 65 dB, >61 dB, >59 dB for λ ₂₁ – λ _23, respectively. The maximum power and wavelength fluctuations are as low as 0.56 dB and 0.01 nm, respectively. The measured laser linewidths ranged from 7.5 to 9.2 kHz, calculated by the β-separation line method based on the measured frequency noise spectra. Nine switchable dual-wavelength operations with different wavelength intervals and two triple-wavelength operations were also obtained. Our proposed fiber laser has great potential in free-space optical communication.