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.
Laser intersatellite link (LISL) offers a large bandwidth, low power loss and high reliability transmission scheme to the satellite communication. With the rise of commercial applications of giant constellations, the LISL technology could be used to build the free-space backbone network to deliver the information all over the world. However, the operational scheme on the new frequency-carrier also brings new issues. The transmission performance is highly dependent on the channel quality, i.e., the laser channel for our case. In this paper, we build up the theoretical model of the transmission channel for the LISL system, considering the sun outage, the doppler frequency shift and the platform vibration as the major noise sources for the free-space laser communication system. The numerical simulation is carried out to quantify the detail impacts from these sources and to define the operational region of the QPSK transmission system. According to our calculations, the large elevation angle, the low frequency shift and the less vibration could improve the transmission performance.
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.
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