We carry out the thoughtful investigations on the end-to-end delay comparison based on seven proposed low-earth-orbit (LEO) satellite constellations, i.e., GlobalStar, Iridium, TeleSat, Kuiper, StarLink, OneWeb with 720 and 1764 satellite nodes, which include two major constellation types-polar and inclined orbits. The discussion focuses on the future satellite operation in space by gradually increasing the number of satellites nodes and analyzes the impact of the constellation scale on the performance of the inter-communication delay. The shortest path algorithm is used to simulate both international (Beijing to New York) and domestic (Beijing to Chengdu) scenarios for each constellation to calculate the shortest transmission delay and its corresponding hop count. Our results show that with the expansion of the number of satellites, the end-to-end delay could touch the floor. The optimized delay is achieved when the number of satellites is close to 1000 for both international and domestic scenarios, which values are 44ms and 6ms, respectively. There is no impressive improvement on the delay performance when further expanding the constellation scale. Moreover, as the number of satellites continue to accumulate, both the long-distance and short-distance communication scenarios, the large-scale star chain clusters aggravate the frequency of inter-star switching, easily leading to unstable transmission delay, which is not conducive to obtaining the best delay benefits. Therefore, for the delay performance-driven service, it is necessary to reasonably optimize the constellation structure to meet the user's communication needs through the appropriate number of satellites in the constellation.
Mode division multiplexing (MDM) technology has become an effective method to further increase the data capacity of optical fiber transmission systems. Few-mode erbium-doped fiber amplifiers (FM-EDFAs) capable of compensating for the link loss have attracted great attention. FM-EDFAs with low differential mode gain (DMG) are desirable for MDM transmission. Since the refractive index distribution and erbium-ion doping profile of a given FM-EDF are usually changeable, ones have to employ the method of optimizing the pumping power, mode and direction etc. to further reduce the DMG. On the other hand, the FM-EDFAs composed of bulky free-space components require too high alignment accuracy to satisfy the engineering needs. Therefore, it is very necessary to build up all-fiber FM-EDFAs with pump optimization. We build up a four-mode or six-mode all-fiber FM-EDFA with low DMG based on the homemade FMEDF from the project (No.2018YFB1801003) and two six-mode mode-selective photonic lanterns (MSPLs). In order to improve the gain of the higher-order mode signals, we utilize three 1480nm pump lasers corresponding to LP21a, LP21b and LP02 modes, respectively. By optimizing the pump powers, the average modal gains are respectively up to 26.2dB with the DMG of 1.5dB for four-mode (LP01, LP11a, LP21a and LP02) signals, and 24.9dB with the DMG of 2 dB for sixmode (LP01, LP11a, LP11b, LP21a, LP21b and LP02) signals.
Soft failure with lower optical signal-to-noise ratio (OSNR) might reduce the quality of the supported services. When the soft failure is detected, the affected existing lightpaths are usually rerouted with alternative paths to avoid the use of the degraded link. However, rerouting without distinguishing between hard failure and soft failure may result in a problem of low utilization of network resources. Unlike hard failure, the degraded link under soft failure can still be used to deliver “shorter” traffic service if the transmission quality requirement can be met. To address this problem, a soft-failure detection method based on deep neural network (DNN) is proposed to detect and localize the failure in elastic optical network. Then, a soft failure aware resources allocation algorithm based on genetic algorithm (SFA-GA) is used for routing and spectrum allocation (RSA) in the network. Simulation results show that 99% accuracy of OSNR estimation can be obtained by the proposed DNN-based scheme for the degraded link under soft failure with estimation errors to be less than 0.5 dB. Based on the estimated OSNR evolution over time, the soft failure can be identified with the degraded link localized. Lower blocking ratio with higher network throughput can also be achieved by the proposed SFA-GA than conventional RSA algorithms. When soft failure exists in the network, the proposed SFA-GA can support the highest traffic load among all five algorithms at any given blocking ratio. At a blocking ratio of 0.01, the SFA-GA allows a traffic load as high as 280 Erlangs, which is about 1.5 times of the commonly used Dijkstra routing plus first fit for spectrum assignment. The traffic load can increase to over 400 Erlangs if a higher blocking ratio of 0.1 is allowed.
The nonlinear propagation characteristics of multiwavelength optical signals in silicon waveguides are investigated for all-optical regeneration. Our experiment and simulation show that the multiwavelength regenerators based on silicon waveguides can be developed with a clock-pump scheme by properly setting the signal and pump power levels, ensuring that the Q-factor degradation induced by the Kerr nonlinear cross talk of the input signals is <1.0 dB and the clock-pump power is no more than the saturated input level related to the nonlinear loss. A three-wavelength regeneration experiment based on the clock-pump four-wave-mixing scheme was demonstrated in the silicon nanowire waveguide, and both the extinction ratio and Q-factor are improved by >3.0 dB for 12.5 Gbit/s on–off keying signals. The feasibility of an eight-wavelength regeneration with the clock pump is also verified by simulation.
The spectrum shift of erbium-doped magneto-optic fiber Bragg grating (Er-MFBG) induced by external magnetic fields is, for the first time, directly measured by the method of the "direct edge detection," and then the effective Verdet constant of −12.42 rad/(T·m) is determined. The theoretical results are in agreement with the experimental data. Our analysis shows the transfer characteristics of the spectrum shift to the transmission power are dependent on the state of polarization and wavelength position of probe light for a given Er-MFBG.
According to the magneto-optic coupling characteristics of magneto-optic fiber Bragg gratings (MFBGs), we present a nonmagnetic equivalent model of a uniformly magnetized MFBG to analyze the optical polarization states output from the MFBG for incident linearly polarized light. The Faraday rotation of transmitted light is enhanced at the edges of a grating bandgap, and then can be applied to the measurement of a magnetic field by employing the wavelength speeding method. The use of apodized MFBGs helps to simultaneously achieve a good linear magnetic field response and extremely small fluctuation of transmittivity. For a typical paramagnetic terbium-doped and apodized MFBG, the temperature coefficient of the Faraday rotation is calculated to be −2.5×10−4 m/K.
The simulation model to bistable characteristics of grating is established by numerically solving the nonlinear coupledmode
equations of Fiber Bragg Grating using 3-stage 4-order semi-implicit Runge - Kutta method. The impact of the
grating parameters and operating point on S-shaped hysteresis loop parameters is researched. The results show that the
area of S-shaped hysteresis loop, bistable operating condition, bistable threshold and dynamic range can be adjusted by
changing the operating wavelength and grating parameters, such as grating length, grating apodization and average
refractive index modulation depth of grating.
Based on the nonlinear coupled-mode equations (NCMEs) of fibre Bragg grating (FBG), all-optically tuned delay is
realized in two linearly apodized FBGs in simulation. The characteristics of tuneable delay under the control of input
optical power are analyzed, and the sensitivity of delay time with optical power is discussed. Results show that the delay
maximum can be obtained at the optimal points in connection with input power, operating wavelength and grating
length.
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