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At theoretical model of three levels describing Holmium-doped fiber lasers (HDFL) pumped by Laser-diodes 1.15 µm is presented. Based on the model, each of the efficiency, output power depending on fiber core radius, fiber length, and ion concentration of active media were investigated, using the MATLAB program (R 2019 a). The results show that at a doped fiber core radius, 2, 5 & 10 µm pumped with a power range of 10 - 100 W got an output power ranging from 5 to 55 W, respectively, while slope efficiency (η) was 31.3%. Output power was 2.7 - 4 W, η = 50% at a fiber length of 0.01 m, and 81.7 - 83 W, η = 19.6% at a fiber length of 0.5 m. The output power ranged from 107.8 to 109 W & η = 4.7% for 3 m fiber length. The threshold power value of Pth = 0.05 W at fiber core radius 2 µm and Pth = 0.006 W at 3 µm was then gradually slightly increased from 4 to 10 µm. As a result, the output power and efficiency are no longer reliant on the diameter of the fiber core. [ABSTRACT FROM AUTHOR]

In this paper, we propose a Hartley-domain direct-detection faster-than-Nyquist (HD-DD-FTN) algorithm for asymmetrically clipped optical single-carrier frequency-division multiplexing (ACO-SCFDM) in optical-wireless communications (OWC). The HD-DD-FTN algorithm includes modified Hartley-domain equalization (HDE) and maximum likelihood sequence detection, which can simultaneously compensate serious high-frequency distortion and eliminate enhanced in-band noise for the bandwidth-limited OWC. Modified HDE can be implemented by real-valued operations, which is appropriate for dealing with real-valued signals. The simulation results show that ACO-SCFDM system using the HD-DD-FTN algorithm is more robust to inter-symbol interference than that using conventional HDE algorithm. Therefore, the bit error rate performance of the ACO-SCFDM system using HD-DD-FTN algorithm is better than that using the conventional HDE algorithm. In conclusion, the ACO-SCFDM system using the HD-DD-FTN algorithm shows great potential for bandwidth-limited OWC.

A simple optical pulse generation scheme for pulsed light detection and ranging (LiDAR) and free-space communications is proposed and experimentally and numerically demonstrated, in which continuous-wave light emitted by a distributed feedback-laser diode (DFB-LD) is modulated by a lithium-niobate Mach-Zehnder intensity modulator to generate optical short pulses, and an Er-doped fiber amplifier (EDFA) is used to boost transmitting light power. Possibilities of intensity modulators for high speed communications being used to generate optical short pulses for low speed pulsed LiDAR are investigated. The influences of bias voltage of intensity modulator and bit rate of modulation signal on the generated optical pulses are discussed. The pulse width obtained by using the return to zero signal with 33% duty cycle on bit rate of 2.5 and 5 Gbit/s is respectively 128.0 and 63.2 ps, and the corresponding nominal accuracy of pulsed LiDAR is respectively 19.2 and 9.5 mm, and low repetition frequency required by pulsed LiDAR is achieved by coding to high-speed modulation signal. The system performance for free-space communications and pulsed LiDAR is evaluated, respectively. We believe that the proposed scheme is suitable for the integrated system of pulsed LiDAR and free-space communications. [ABSTRACT FROM AUTHOR]

In this paper, we propose a Hartley-domain direct-detection faster-than-Nyquist (HD-DD-FTN) algorithm for asymmetrically clipped optical single-carrier frequency-division multiplexing (ACO-SCFDM) in optical-wireless communications (OWC). The HD-DD-FTN algorithm includes modified Hartley-domain equalization (HDE) and maximum likelihood sequence detection, which can simultaneously compensate serious high-frequency distortion and eliminate enhanced in-band noise for the bandwidth-limited OWC. Modified HDE can be implemented by real-valued operations, which is appropriate for dealing with real-valued signals. The simulation results show that ACO-SCFDM system using the HD-DD-FTN algorithm is more robust to inter-symbol interference than that using conventional HDE algorithm. Therefore, the bit error rate performance of the ACO-SCFDM system using HD-DD-FTN algorithm is better than that using the conventional HDE algorithm. In conclusion, the ACO-SCFDM system using the HD-DD-FTN algorithm shows great potential for bandwidth-limited OWC. [ABSTRACT FROM AUTHOR]

Abstract: A method of on-ground simulation of optical links for free-space communications by Fourier-transform, image magnification and wavefront sampling with limited apertures was proposed. An optical simulator for free-space laser links was designed, which has the range of equivalent links distances from 160,000km to 25km. It can be used for the evaluation of communication performances of optical links in space, particularly for measuring the bit error rate under a constant transmission distance or for testing the equivalent transmission distance under a constant bit error rate. System analysis indicates that the wavefront aberrations of Fourier-transform lens have the main influence on the measurement results. But these effects can be corrected only if the RMS value of wavefront aberrations of Fourier-transform lens on the work aperture is known. Simulation experiment further shows that this method will be feasible by providing excellent optical devices and fine calibration accuracy and by overcoming the environmental disturbances. [Copyright &y& Elsevier]

• Based on a far-field asymptotic expression of diode laser beams, the collimation characteristics of a diode laser beam are investigated. • A method for designing a collimating system of a laser diode is proposed, which is confirmed by experiment. • It is shown that a circular beam spot could be obtained in the far field zone, only if the beam divergence angles have the same value in the two orthogonal directions. Based on a far-field asymptotic expression of diode laser beams, the collimation characteristics of a diode laser beam are investigated. In this paper we propose a method for designing a collimating system of a laser diode and do experiment to confirm it. It is shown that the collimated beam divergence angles in the fast- and slow-axis directions are different, if the beam sizes in the two orthogonal directions near the collimating system are the same. It is also shown that a circular beam spot could be obtained in the far field zone, only if the beam divergence angles have the same value in the two orthogonal directions, although the beam is not circularized near the collimating system. [ABSTRACT FROM AUTHOR]