Simultaneous measurement of orbital angular momentum spectra in a turbulent atmosphere without probe beam compensation

In free-space optical (FSO) communications, the orbital angular momentum (OAM) multiplexing/demultiplexing of Bessel beams perturbed by atmospheric turbulence is of great significance. We used the Gerchberg-Saxton algorithm without a beacon beam to compensate for the aberrant helical phase of the Bessel beam distorted by the turbulent atmosphere. The optical vortex Dammann axicon grating was applied for the simultaneous measurement of the intensities of the demodulated spectra of the OAM modes of the Bessel beams disturbed by atmospheric turbulence. The experimental results demonstrate that the distorted phase of the Bessel beam can be compensated and the mode purity of the target OAM mode is enhanced from 0.85 to 0.92 in case of weak turbulence. Our results will improve the quality of the OAM modes of Bessel beam (de)multiplexing in FSO communication systems

Performance of Orbital Angular Momentum Communication for a Non-Uniformly Correlated High-Order Bessel–Gaussian Beam in a Turbulent Atmosphere

We derived the formula for the detection probability, signal-to-noise ratio (SNR), and average bit error rate (BER) for the signal orbital angular momentum (OAM) state carried via non-uniformly correlated high-order Bessel–Gaussian beam propagation in a turbulent atmosphere. The wavelength, receiver aperture, beam width, strength of the turbulent atmosphere, and topological charge effect on detection probability, SNR, and average BER of the signal OAM state were demonstrated numerically. The results show that the signal OAM state with low topological charge, a small receiver aperture, a narrow beam width, and a long wavelength can improve the performance of optical communications systems under conditions of weak atmospheric turbulence. Our results will be useful in long-distance free space optical (FSO) communications

Designing Limited-Stop Transit Service with Fixed Fleet Size in Peak Hours by Exploiting Transit Data

Efficiently designed limited-stop transit service is an attractive way to respond to high commuter travel demand in which trips concentrate on a few origin–destination pairs during peak hours. Such service is redesigned in many metropolises in China. Some research has dealt with this situation; bus fleet size was assumed to be unlimited, and the research was concerned with the average daily passenger flow rather than the specific average peak hour travel demand. In contrast to previous work, this paper presents an approach to design limited-stop transit service with the existing available fleet size from current normal service and focuses only on peak hour travel demand extracted through exploitation of transit data. First, a model for limited-stop service was proposed to minimize user costs through existing fixed fleet size. A heuristic algorithm was developed to search the transit line structure for limited-stop service instead of selecting lines from the predefined set. Next, a case in Chengdu, China, was tested. The results indicate that up to 9.32% of total travel time can be saved with the fixed fleet size when limited-stop transit service is applied. Finally, different proportions of commuter flow and different travel behaviors are discussed to illustrate the performance of limited-stop service for different scenarios

Influence of Real-Time Heating on Mechanical Behaviours of Rocks

The rock mechanical properties under the effect of high temperature present a great significance on underground rock engineering. In this paper, the mechanical properties of sandstones, marbles, and granites under real-time heating were investigated with a servo-controlled compression apparatus. The results show that mechanical behaviours of all the three types of rocks are influenced by real-time heating to different degrees. Due to thermal cracking, the uniaxial compressive strengths decrease as the heating temperature rises from room temperature to 400°C. Above 400°C, the sandstone exhibits a significant increase in UCS because of the sintering reaction. The sintering enlarges the contact area and friction between crystal grains in the sandstone, which strengthens the bearing capacity. For marbles, the UCS continues to decrease from 400°C to 600°C due to thermal cracking. However, the carbonate in the marble begins to decompose after 600°C. The generated particles would fill the cracks in the marble and increase the strength. For granites, their UCS presents a sharp decline after 400°C because of thermal cracking. For all rock elastic modulus, they present a decreasing trend, and this indicates that the rock’s ability to resist deformation gradually weakens under the effect of temperature. In general, rock mechanical behaviours under real-time heating differ from those in normal situations, and use of the parameters presented here is important for underground rock engineering related to high temperature and can improve the precision in theoretical and numerical analysis

M2-factor of coherent and partially coherent dark hollow beams propagating in turbulent atmosphere

Analytical formula is derived for the M2-factor of coherent and partially coherent dark hollow beams (DHB) in turbulent atmosphere based on the extended Huygens-Fresnel integral and the second-order moments of the Wigner distribution function. Our numerical results show that the M2- factor of a DHB in turbulent atmosphere increases on propagation, which is much different from its invariant properties in free-space, and is mainly determined by the parameters of the beam and the atmosphere. The relative M2-factor of a DHB increases slower than that of Gaussian and flat-topped beams on propagation, which means a DHB is less affected by the atmospheric turbulence than Gaussian and flat-topped beams. Furthermore, the relative M2-factor of a DHB with lower coherence, longer wavelength and larger dark size is less affected by the atmospheric turbulence. Our results will be useful in long-distance free-space optical communications.</p

0-360 Degrees angular measurements using spatial displacement

The angle measurement is one of the fundamental measurement techniques required for a large number of applications. Many mechanical and optical measurement methods accomplish this task, requiring contact with the object or interference using multiple laser beams. We introduce a measurement for a given arbitrary angle accomplished by the conversion of the angle measurement to a spatial displacement measurement. The resolution of the angular measurement is a microradian, and it is limited only by the resolving power of a pixel. This measurement is almost independent of the specific wavelength, degree of coherence, and measurement distance. Moreover, our method realizes a non-contact and nondestructive angular measurement within the range of 0°-360°. We anticipate it to have important applications in optical communication systems and industry.</p

Signal quality enhanced and multicast transmission via cylindrical vector beams with adaptive optics compensation in free space optical communications

Signal quality is critical in free space optical (FSO) communications. Many technologies and methods are applied to enhance the signal quality of an optical communication system, which might require one of the methods employing structured light. We introduce structured light of cylindrical vector beams with adaptive optics compensation to improve signal quality in FSO communications. The multiplexing/demultiplexing of coaxial cylindrical vector beams is used to increase the capacity of the communication channel. Adaptive optics compensation and cylindrical vector beams are applied to mitigate influence of the turbulent atmospheric disturbances and improve the signal quality. An optical vortex grating is employed to demultiplex signal beams and then send the signals to a receiver plane to measure the bit-error-rate. Consequently, the signal quality of the cylindrical vector beams multiplexing/demultiplexing in FSO communication systems improves.</p

Propagation of Optical Coherence Vortex Lattices in Turbulent Atmosphere

Propagation properties in the turbulence atmosphere of the optical coherence vortex lattices (OCVLs) are explored by the recently developed convolution approach. The evolution of spectral density distribution, the normalized M 2 -factor, and the beam wander of the OCVLs propagating through the atmospheric turbulence with Tatarskii spectrum are illustrated numerically. Our results show that the OCVLs display interesting propagation properties, e.g., the initial Gaussian beam distribution will evolve into hollow array distribution on propagation and finally becomes a Gaussian beam spot again in the far field in turbulent atmosphere. Furthermore, the OCVLs with large topological charge, large beam array order, large relative distance, and small coherence length are less affected by the negative effects of turbulence. Our results are expected to be used in the complex system optical communications