We investigate and analyze the transmission performance of free-space optical (FSO) communication systems at three different working wavelengths, 1.55, 4, and 9 μm, under the combined effects of weather conditions (rain, snow, fog, and haze) and atmospheric turbulence, which is modeled by the gamma-gamma turbulence model. The simulation system of FSO communication based on quadrature phase-shift keying (QPSK) modulation format with a data transmission rate of 10 Gbps is built using OptiSystem software. The results show that the working wavelength has less influence on the transmission performance of the FSO communication system under rainy weather and turbulence conditions. In snowy and turbulent conditions, the short-wavelength laser has a stronger transmission advantage. In fog and turbulence situations, the transmission performance of the FSO communication system is significantly improved with the increase of wavelength. Under the combined influence conditions of haze and medium turbulence, the transmission distance of 9-μm laser is 2.67 and 1.56 times that of 1.55- and 4-μm lasers, respectively, when the bit error rate is 10−9. The results can provide an important reference for the design and optimization of practical FSO communication systems.
We propose a simulation scheme for cascaded four-wave mixing communication transmission characterization in the mid-infrared band. Simulations are performed using OptiSystem software. Signal light and pump light with wavelengths of 2052 and 2050 nm, respectively, passed through a highly nonlinear fiber, and significant cascade four-wave mixing phenomenon occurred, and the signal loaded on the signal light was successfully transmitted to the idler light generated by the cascade four-wave mixing effect. The performance of the signals carried in each idler light in both fiber and free-space channel was tested by eye diagrams and BER tests. At a pump power of 24 dBm, eight optical carriers were obtained the could meet the communication requirements, realizing a total transmission rate of 80 Gb/s. This is the first time that a simulation analysis of cascaded FWM multicast in the 2 μm band has been reported. The research results can promote the development of optical communication technology in the mid-infrared band and provide a reference for the establishment of high-speed multiplexing systems.
In the last few years, all-optical signal processing based on 2D materials has been a hot topic because of its ultra-fast optical response, broadband optical absorption, and tunable optoelectronic characteristics. However, there are few reports about all-optical wavelength conversion concentrate on the 2μm communication band in the existing research for the limits of small nonlinear coefficient, serious linear and nonlinear loss, or small conversion bandwidth. In our work, four-wave mixing in the 2μm band is realized by fabricating graphene modified micro/nano optical fiber devices, which can achieve the -46.21dB wavelength conversion efficiency. This work will contribute to the practical application of 2μm band wavelength conversion.
The dissipative soliton resonance (DSR) pulses are demonstrated experimentally in an all polarization-maintaining (PM) thulium-doped mode-locked fiber laser. The mode-locked operation is achieved using of the nonlinear amplifying loop mirror mechanism (NALM). Each loop of the apparatus includes an independently controlled amplifier and a section of gain fiber. The experiment is carried out at a central wavelength of 1969.8 nm and 3.5 ns as the maximum output pulse width. The time domain and spectrum characteristics of output pulse are analyzed with different positions of PM-1950 fiber in the NALM loop. In addition, the DSR pulse properties are experimentally verified by changing the length of the PM-1950 fiber spliced into the NALM loop. We have achieved environmentally stable mode-locked pulses at 3, 2.1, and 1.62 MHz corresponding to the maximum pulse energies of 6.8, 20, and 29.1 nJ. The output pulse is stable and robust at different ambient temperatures.
We present the generation and optimization of square-wave noise-like pulses (NLPs) in a mode-locked Tm-doped fiber laser. Mode-locking operation around the 2-μm band is achieved by a nonlinear amplifying loop mirror. To optimize the output performance, the figure-eight cavity is modified by employing a polarization-dependent isolator in a unidirectional loop, and the cavity length is only 17.2 m. First, by employing a cavity with pure anomalous dispersion, a conventional soliton can evolve into a square-wave NLP by properly setting the pump power and polarization controllers. The pulse energy of the fundamental-frequency operation can be varied from 2.29 to 3.4 nJ. Using an ultrahigh-numerical-aperture fiber to reduce the net dispersion to −1.033 ps2, the 3-dB bandwidth of the spectrum is broadened to 14.78 nm, and the duration of the autocorrelation spike is only 421 fs. The maximum single-pulse energy can increase up to 4.97 nJ. Due to dispersion management mechanism, the threshold and output power are also significantly improved.
We have experimentally obtained dispersion-managed solitons with sidebands in a passively mode-locked thulium-doped fiber laser. The stable single soliton with sidebands can be converted into two soliton pulses at the pump power of 867 mW with appropriate settings of the polarization controllers (PCs). By increasing the pump power and cautiously adjusting the PCs, the three, four, and five soliton pulses with nonuniform intensity operate with stability in the cavity due to the global soliton interaction caused by unstable continuous waves. Furthermore, the soliton bunch can be observed at the pump power of 1 W. The position of solitons in the soliton bunch is random with a fixed separation that is controllable by changing the linear phase delay. Our work gives insight into the dynamics of multipulse dispersion-managed solitons in a 2-μm mode-locked fiber laser.
We experimentally studied a passive mode-locked square-wave fiber laser with net-normal dispersion by using a nonlinear amplifying loop mirror. The output pulse width of the square-wave pulse (SWP) can be extended with the increasing of the pump power as well as single pulse energy. The SWP can be amplified without shape distortion when the mode-locked laser is injected into an erbium-ytterbium codoped fiber amplifier. Then, by inputting the amplified SWPs into a single mode fiber (SMF), a supercontinuum (SC) can be observed with the spectral range of about 1550 to 1850 nm. Broader and flatter SC with the 20-dB spectral range of about 1550 to 1950 nm can also be generated using a segment of dispersion-shifted fiber instead of SMF.
A simple approach to generate passively harmonic mode-locked pulse trains in thulium-doped fiber laser based on nonlinear polarization rotation is proposed and demonstrated. Three different ways of mode-locked techniques have been employed in our structure to generate passively high-order harmonic mode-locked pulse trains; 128th-order passively harmonic mode-locked pulse train is achieved in the experiment and the repetition rate is 406.8 MHz. With the increase of the pump power, multiwavelength output can be tuned. A segment of dispersion compensation fiber is used to compensate the dispersion in the cavity; thus, the single pulse width is compressed from 617 to 48 ps.
To suppress the interference of the target detecting in the turbid medium, a kind of polarization detection technology based on Curvelet transform was applied. This method firstly adjusts the angles of polarizing film to get the intensity images of the situations at 0°,60° and 120°, then deduces the images of Stokes vectors, degree of polarization (DOP) and polarization angle (PA) according to the Mueller matrix. At last the DOP and intensity images can be decomposed by Curvelet transform to realize the fusion of the high and low coefficients respectively, after the processed coefficients are reconstructed, the target which is easier to detect can be achieved. To prove this method, many targets in turbid medium have been detected by polarization method and fused their DOP and intensity images with Curvelet transform algorithm. As an example screws in moderate and high concentration liquid are presented respectively, from which we can see the unpolarized targets are less obvious in higher concentration liquid. When the DOP and intensity images are fused by Curvelet transform, the targets are emerged clearly out of the turbid medium, and the values of the quality evaluation parameters in clarity, degree of contract and spatial frequency are prominently enhanced comparing with the unpolarized images, which can show the feasibility of this method.
In this paper, a segment of thulium-doped fiber is pumped by a 976nm laser diode.Broadband gain
at centerwavelength of 1953nm is achieved. The maximum amplified spontaneous emission
bandwidth is 8nm. Simultaneously, self- oscillation of wavelength spacing 0.073nm is
observed. The output power and self- oscillation modes increase with pump power increasing and
wavelength spacing of self-oscillation is unchanged.The relation between fiber length and output
power at pump power 400mW is analyzed. Higher output power can be abtained by selecting a
appropriate fiber length when pump power is unchanged. Through the experiment, a weak
absorption band of thulium-doped fiber near 976nm is verified. The structure can be used as
narrow linewidth broadband source near 1950nm with the characteristic of low cost, simple
structure and good stability.
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