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.

We present a method in this Letter to generate optical vortices with tunable orbital angular momentum (OAM) in optical fibers. The tunable OAM optical vortex is produced by combining different vector modes ${\mathrm{HE}}_{2,m}^{\text{even}}$ (${\mathrm{HE}}_{2,m}^{\text{odd}}$) and ${\mathrm{TE}}_{0,m}$ (${\mathrm{TM}}_{0,m}$) when $l=1$ or combining ${\mathrm{HE}}_{l+1,m}^{\text{even}}$ (${\mathrm{HE}}_{l+1,m}^{\text{odd}}$) and ${\mathrm{EH}}_{l-1,m}^{\text{odd}}$ (${\mathrm{EH}}_{l-1,m}^{\text{even}}$) when $l>1$ with a $\pi /2$ phase shift. The vortex can be regarded as a result of overlapping two orthogonal optical vortex beams of equal helicity but opposite chirality with a $\pi /2$ phase shift. We have experimentally demonstrated the smooth variation of OAM from $l=-1$ to $l=+1$ by adjusting a polarizer at the output end of the fiber.

We presented a simple method for measuring the mode purity of linearly polarized orbital angular momentum (OAM) modes in optical fibers. The method is based on the analysis of OAM beam projections filtered by a polarizer. The amplitude spectrum and phase spectrum of a data ring derived from the beam pattern are obtained by Fourier transform. Then the coefficients of the mixed electric field expression can be determined and the mode purity can be obtained. The proposed method is validated and it is experimentally demonstrated in a two-mode fiber.

We propose an effective approach to generate tunable orbital angular momentum (OAM) beam based on all-fiber fused coupler consisting of a single mode fiber (SMF) and a two-mode fiber (TMF). The fundamental mode in the SMF is directly coupled to the LP11 mode group (TE0,1, HEeven,odd 2,1, TM0,1) by appropriately phase matching the modes in the fiber coupler. The experimental results demonstrate that the OAM beam is produced by combining HEeven 2,1 and TE0,1 (or HEodd 2,1 and TM0,1) with a π/2 phase shift and the OAM topological charge could be tuned from l = -1 to l = +1 by adjusting a polarizer set at the output end of fiber.