Gravitational field is a kind of vector field,and the description of general relativity to gravitational force problem is very effective.This paper deals with the equivalence principle and the principle of general covariance in general relativity,discusses Einstein’s gravitational field equations,researches on the principle of relativity,time dilation,contraction of length in gravitational field.
Abstract In recent years, with the continuous progress of laser technology, fiber laser has been developed rapidly. Compared with the traditional laser, fiber laser has the advantages of good beam quality, small volume, easy integration, high luminous efficiency, strong anti-interference ability and good heat dissipation. As a kind of fiber laser, thulium doped fiber laser not only inherits the above advantages, but also has the rich energy level structure and spectral characteristics of thulium ion, which determines that thulium doped fiber laser plays an important role in the research field of long wavelength fiber laser. This paper discusses the influence of the length and concentration of doped fiber on the output of high power laser system. For a given laser system, all parameters have an optimal value. In order to make the high power thulium doped fiber laser work in the best state, we need to carefully select these parameters. Based on the rate equation, the relationship between the dopant concentration and the length of the optical fiber is given in theory. The experimental results show that the optimal fiber length is different for different pump power. The higher the pump power, the larger the optimal fiber length. The experimental results have guiding significance for the manufacturing technology of high power fiber lasers.
Survivability is the capability of a network to maintain service continuity in the presence of faults within the network. In this paper, we presented the new survivability method and discussed the capability of survivability in optical network. The survivability in WDM networks is implemented using protection and restoration techniques. Protection is a static mechanism to protect against failure, where the resource for both the primary and the backup light-paths are reserved prior to the data communication. Restoration on the other hand, is a dynamic mechanism where the backup light-path is not set up until the failure occurs. Survivability using these techniques is usually provided to handle single link failures in the core network.
In this letter, we propose a novel approach for optical single-sideband (SSB) generation by utilizing four-wave mixing Bragg scattering (FWM-BS). The scheme comprises three stages: the initial FWM-BS stage, the spectral phase control stage, and the final FWM-BS stage. Constructive or destructive interference can occur between the sidebands generated in the two FWM-BS stages, depending on the spectral phase mask applied between the stages. By applying an appropriate phase mask, one sideband can be eliminated by destructive interference, while the other sideband is magnified by constructive interference, thus realizing SSB generation. Both upconverted and downconverted SSB generations can be achieved. To verify the scheme, we experimentally demonstrate 50-GHz optical frequency upconversion and downconversion separately for multi-channel 16-Gbaud QPSK signals.
According to the time-space metric, the singularity problems in the general relativity are studied. The famous singularity of Schwarzschild metric in the general relativity is investigated. Pointing out that the singularity revealed the defects of the general relativity in theory and was not the intrinsic property in the nature world. Some singularity problems in the time-space metric are due to the incorrect selection of time-space coordinates, and therefore get the false singularity. Only introducing appropriate time-space transformation, one can remove the singularity. In the general relativity, the methods of eliminating the singularity at r=r_s through the three different time-space coordinate transformation are given out. At the same time,the coordinate speeds of photon under three different time-space metrics were discussed.
Wavelength multicasting is one of the desirable functional blocks in future reconfigurable optical networks. The four-wave-mixing Bragg scattering-based wavelength multicasting can provide modulation format transparency and polarization-independent operation with comparatively low system complexity. One crucial challenge of this scheme is that the conversion efficiencies of the red-shifted and blue-shifted generated copies are imbalanced due to non-uniform phase matching conditions. The asymmetry is even worse when the generated copies are spread over a large spectral range. To solve this problem, we utilize gain-transparent stimulated Brillouin scattering in the nonlinear fiber to introduce a spectrally localized nonlinear phase to reduce the imbalance, resulting in enhanced performance of the wavelength multicasting system.
Abstract With the rapid development of optical communication network and related technology, the frequency resource of light wave is becoming more and more insufficient, which requires new light sources to support the communication system.With the continuous development of fiber laser technology, Tm-doped fiber laser appears, which brings new vitality to the optical communication network with insufficient frequency resources. Because Tm -doped fiber laser can provide multiple frequencies of optical wavelength resources, it has attracted more and more attention. The development of rare earth doping technology and double cladding pumping technology has laid a solid foundation for the design of Tm-doped fiber laser with excellent performance. In this paper, the Tm-doped fiber laser is studied. The effect of each element in the Tm-doped fiber laser on the laser is analyzed. The influence of the reflectivity of the mirror on the performance of the whole laser is discussed. The simulation results show that the reflectivity of the mirror will seriously affect the output power of the laser, which is of great significance for the design of fiber lasers.
We demonstrate a tunable optical delay that surpasses the tuning speed limit of the conventional slow-light-based optical delay. A novel nonlinear optical coupler, implemented by the four-wave mixing (FWM) Bragg scattering process, is utilized to perform destructive interference of the slow-light delayed signal pulse and a nondelayed reference pulse. As a result, the Brillouin-induced frequency-dependent phase shift, as well as the group delay of the synthesized pulse, is amplified. The group delay amplification factor, determined by the coupling ratio of the nonlinear optical coupler, can be tuned through varying the FWM pump power to provide an ultrafast response. Our experimental result demonstrates that an initial 6.2 ns Brillouin-induced optical delay can be amplified and rapidly tuned within the range of -5.2 to 27.2 ns.
Abstract Rare earth doped optical fibers have important applications in fiber lasers, amplifiers and sensors. The Tm-doped fiber laser made with Tm-doped fiber as gain medium can be used in optical communication system. Because the fiber itself has a cylindrical waveguide structure and the core diameter of the fiber is very small, it is easy to achieve high energy density pumping in the fiber. Fiber laser has low laser threshold and good heat dissipation performance, and its core diameter matches well with the communication fiber, forming the integration of transmission fiber and active fiber, which can be an important basis for all-optical communication. However, due to the dispersion of Tm-doped fiber laser, it will lead to the broadening of laser pulse and the degradation of laser quality, which seriously affects the quality of laser communication. In this paper, the causes of dispersion are analyzed, and a dispersion adaptive control system is designed. The experimental results show that the proposed dispersion compensation system can compensate the dispersion produced in the optical fiber laser to a certain extent, so that the laser pulse can return to normal state.This is of great significance for optical communication systems.
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