Rui Hou

Influence on the Bit Error Ratio of the Optical Satellite Communication System under Space Radiation Environment

To improve the space anti-radiation performance of the optical satellite communication system, the research of the influence on its system bit error ratio(BER) under space radiation environment was developed. The radiation-induced BER model was established, based on which the BER numerical calculations after the radiation damage of either optical source or optical detector were developed. The calculation results have proved that both γ and proton radiations can degrade the system BER by several levels, some suitable radiation shielding equipments are further needed.

Impact of pointing errors on performance of an intersatellite microwave photonics link with an optical preamplifier under dual-tone modulation.

The intersatellite microwave photonics link with an optical preamplifier is affected by third-order intermodulation distortion under dual-tone modulation and pointing errors due to beam wander, which would greatly degrade the link performance. An exact analytical expression for signal-to-noise and distortion ratio (SNDR) is derived considering the signal fade caused by the pointing errors of transceiver. It is shown that, given the desired SNDR and the rms random pointing jitter, an optimum modulation index of Mach-Zehnder modulator exists that minimizes laser output power. Moreover, an optimized model for laser output power and modulation index is established. The effects of the optical preamplifier gain and noise figure on the optimum link performance are also examined. Numerical results show that the minimum laser output power required to achieve the desired SNDR is more sensitive to the preamplifier noise figure. For an SNDR of 20 dB, doubling the preamplifier noise figure results in an 8.95 dB increase in minimum laser output power at the rms pointing jitter of 0.5 μrad.

A novel two-layered optical satellite network of LEO/MEO with zero phase factor

Optical satellite communication has advantages over the RF satellite communication in capacity, speed, secrecy and anti-interference. Space information superhighway composed of GEO/MEO/LEO satellites which are linked by optical inter-satellite links (OISL) would play important roles in communication, navigation, martial recon, remote sensing and deep space exploration. A novel LEO/MEO two-layered optical satellite network with global coverage is designed with the street-of-coverage technique. The layer of LEO satellites is the walker-delta constellation with zero phase factor and has the quasi-stable logical structure of Mesh. Equatorial and polar orbit satellites constitute the layer of MEO satellite, which can cover LEO layer effectively. Rising satellites of the system can provide 99.9% of coverage for the whole globe and 100% for China averagely. The network can provide a minimum elevation angle at 21° with single-satellite visibility for China, and at 16° with the dual-satellite visibility. Simulation results of the inter-satellite links show LEO constellation of zero phase factor outperforms non-zero phase factor constellations in azimuth angle, elevation angle and distance, which is very fit for the OISL. Simulation results of the communication performance indicate a little difference between short wave band and long wave band at low communication speed (several Gbit/s); however, the wave band at 1550 nm is a better choice at high communication speed (100 Gbit/s).

High power high brightness laser source for material processing through incoherent beam combination

With the continuous increase of output power of double cladding fiber lasers, more effort is put into the researches of the technique of fiber laser beam combination, especially for incoherent laser beam combination because it is easier and with better system stability. Once high power output laser beam is achieved, there would be broad applications in industry, especially for manufacturing and material processing. The combination systems coupling efficiency plays an important role in determining the output power. Through theoretical analysis and numerical simulations, it has been proved that lower lateral off-set and higher grating period would be favorable, also an optimum spot radius exists which corresponds to a maximum value of coupling efficiency. Although lower focal length is helpful in improving the coupling efficiency, there is a contradiction that it makes a narrower fiber array width, which would limits the number of fiber lasers that could be utilized. Thus a moderate value of 20cm is chosen. Based on such optimized parameters, the beam quality M2 is around 2, also a method of two parallel gratings is introduced, which ensures the M2 factor to be around 1. Such combined fiber laser would be of great potential applications in manufacturing and material processing.

Analysis of charge transfer inefficiency of CCD equipment under proton radiation

The charge coupled device(CCD) is the key component of beacon light subsystem in optical satellite communication system. The influences of space radiation environment on CCDs charge transfer efficiency(CTE) and its dark current are theoretically analyzed. Based on the analysis the induced CTE degradations under some typical operation temperatures and particle fluences are numerically calculated. The calculation results have proved that adding shielding layers and temperature controlling unit are both the efficient ways to improve the radiation hardness of optical satellite communication system.

The optimization of incoherent fiber laser beam combination

With the continuous increase of output power of double cladding fiber lasers, the researches of the techniques of fiber laser beam combination have become a new focus. With the inherent characters of small volume, light weight, good beam quality and system stability. Once the high power output laser was achieved, there would be broad applications in industry, scientific researches and military field. Incoherent beam combination is much easier to be realized compared with coherent beam combination, and with better system stability, thus it became an promising technique to achieve small volume and high power laser source. In an incoherent fiber laser beam combination system, the transform lens and diffractive grating play an important role in determining the coupling efficiency. Through theoretic analysis and numerical simulations, it has been proved that the optimum focal length of transform lens should be around 20cm in order to ensure a high system coupling efficiency under some selected simulation parameters. Also the lower frequency of diffractive grating is preferred, and a contradiction is present that is the higher frequency of diffractive grating is favorable in the consideration of maximum fiber array width, thus a moderate value was chosen as 200mm-1 with comprehensive consideration. The lower grating thickness is helpful in improving the diffractive efficiency, while it faced the problem of heat resistance and manufacturing difficulty, therefore the grating thickness between 2mm and 4mm is preferred. Under such optimized parameters, the coupling efficiency would be 70% or even higher with small deviation angle and wavelength. An example of effective jamming distance of combined fiber laser to visible light silicon CCD has been given, whose results had proved the application foreground of combined fiber laser.