Qiqi Han

Log-amplitude variance for a Gaussian-beam wave propagating through non-Kolmogorov turbulence

In the past decades, both the increasing experimental evidences and some results of theoretical investigation on non-Kolmogorov turbulence have been reported. This has prompted the study of optical propagation in non-Kolmogorov atmospheric turbulence. In this paper, using a non-Kolmogorov power spectrum which owns a generalized power law instead of standard Kolmogorov power law value 11/3 and a generalized amplitude factor instead of constant value 0.033, the log-amplitude variances for a Gaussian-beam wave are derived in the weak-fluctuation regime for a horizontal path. The analytic expressions are obtained and then used to analyze the effect of spectral power-law variations on the log-amplitude fluctuations of Gaussian-beam wave.

Research on Bit Error Rate in the Presence of Local Wavefront Aberration in Intersatellite Laser Communications

Local distortions in lenses of transmitter optical system could further deteriorate the performance of intersatellite laser communications (lasercom). Bit error rate (BER) for on-off keying system is investigated considering the combined influence of local distortion and pointing error, without considering the noise of the detector. It is shown that BER varies regularly with the characteristic parameters of local distortion. Peak-to-valley value that is usually used to evaluate lens machining precision is determined, and the key principles for selecting lenses are given. This paper can contribute to the lasercom system design.

Plane wave coupling into single-mode fiber in the presence of random angular jitter

Efficient coupling of coherent plane waves into single-mode fibers is a key technology for free-space optical communication at 1550 nm. Here we deal with the influence of random angular jitter on a plane wave to single-mode fiber coupling. The expression of mean-coupling efficiency in the presence of random jitter is handled in the pupil plane. First, an analytical expression of the mean-coupling efficiency is derived for the zero bias error case. Then the bias error was taken into account. By minimizing the coupling efficiency penalty, the optimum value of design parameter beta of fiber-coupled optical systems in the presence of random jitter is obtained. The results obtained here will be useful in facilitating parametric estimation and optimization of fiber-coupled optical systems.

Approach to improve beam quality of inter-satellite optical communication system based on diffractive optical elements

For inter-satellite optical communication transmitter with reflective telescope of two-mirrors on axis, a large mount of the transmitted energy will be blocked by central obscuration of the secondary mirror. In this paper, a novel scheme based on diffractive optical element (DOE) is introduced to avoid it. This scheme includes one diffractive beam shaper and another diffractive phase corrector, which can diffract the obscured part of transmitted beam into the domain unobscured by the secondary mirror. The proposed approach is firstly researched with a fixed obscuration ratio of 1/4. Numerical simulation shows that the emission efficiency of new figuration is 99.99%; the beam divergence from the novel inter-satellite optical communication transmitter is unchanged; and the peak intensity of receiver plane is increased about 31% compared with the typical configuration. Then the intensy patterns of receiver plane are analyzed with various obscuration ratio, the corresponding numerical modelling reveals that the intensity patterns with various obscuration ratio are nearly identical, but the amplify of relative peak intensity is getting down with the growth of obscuration ratio. This work can improve the beam quality of inter-satellite optical communication system without affecting any other functionality.

Electron radiation effects on InAs/GaAs quantum dot lasers

The InAs/GaAs quantum dot laser diodes and corresponding quantum dot samples are irradiated by 1 MeV electron. The laser performance and quantum dot photoluminescence intensity at room temperature are enhanced over a fluence range of 4 × 1013 cm−2. The radiation-induced defects increase the efficiency of carrier transfer to the quantum dots, which results in the improvement of photoluminescence performance under low level displacement damage. The contact resistant of quantum dot lasers decreases because the ohmic contact is also improved by electron irradiation.

Statistical distribution of fiber-coupling efficiency in the satellite-to-ground downlink

In the satellite-to-ground downlink lasercom systems, efficient coupling of atmospherically distorted spatial light into single-mode fibers is a key technology for fiber-coupled optical receiver. Statistical distribution of fiber-coupling efficiency is important to analyze the performance of a receiver. We use the simulation of beam propagation to examine statistical distribution of fiber-coupling efficiency for the satellite-to-ground downlink. The distributions of the fiber-coupling efficiency for different numbers of speckles are obtained, and the cumulative density functions of the fiber-coupling efficiency are also given. In order to validate the accuracy of numerical simulation method, the dependence of average fiber-coupling efficiency on the ratio of lens size to speckle size is attained, which is highly consistent with the prior theoretical works. The results will be helpful in the design of the satellite-to-ground downlink lasercom system based on fiber-coupling technology.

Pointing error due to temperature distribution of SiC reflectors in intersatellite laser communications

Wave-front aberrations due to thermal deformation of elliptical reflectors in periscopic laser communication terminals are studied, and are fitted by Zernike elliptical polynomials. The relationship between pointing error and temperature distribution of elliptical reflectors in intersatellite laser communication systems is researched. The back-fixing method is introduced for fixing the elliptical reflectors, and is proven to reduce pointing error compared to the traditional around-fixing method using a press board. It is shown that the difference between uniform temperature and reference temperature has a much stronger influence than the temperature gradient. For the back-fixing method, the pointing error changes periodically as the uniform temperature changes with a period value of 1.6 degrees C, and the maximum pointing error value is 16.7murad. The result also tells us that, if the difference between uniform temperature and reference temperature is within 1.0 degrees C, there is no pointing error caused by back-fixed reflectors. We hope this work can contribute to the thermal control of elliptical reflectors in intersatellite laser communication terminals.

Impact of random angular jitter on fiber-coupled differential phase-shift keying receivers with Mach–Zehnder interferometer demodulation

In an optically preamplified differential phase-shift keying (DPSK) receiver with Mach–Zehnder interferometer (MZI) demodulation, it is essential to couple free space light into a single-mode fiber, by which the received optical signal can be guided into the amplifier and MZI. Random angular jitter has a profound impact on fiber coupling efficiency, in that it would significantly deteriorate the receiver performance. In this paper, the average bit error rate (BER) of fiber-coupled DPSK receivers is obtained in the presence of random angular jitter. It is shown that the degradation is significant when the random angular jitter normalized by the mode field radius is larger than 0.3. We further analyze the impact of other factors, including central obstruction and optical filter transfer functions. Our results show that the super-Gaussian filter outperforms other filters and leads to lower average BER at a fixed optical signal-to-noise ratio. This work can be used to improve the design of fiber-coupled DPSK receivers.

Wave-front distortion and its effect on inter-satellite laser communication systems

Wavelet model is built to depict the diversified wave-front distortion. Compared to Gaussian model, the effect on pointing error is analyzed. For it is difficult to detect the wave-front distortion to the receiver, especially for long distance, a near field center power measurement method is proposed.

Celestial background noise analysis for laser intersatellite links

The celestial background (CB) has a potential significant influence on the laser intersatellite links (ISLs). A simple method to analyze the power and statistical distribution of the CB noise received by optical terminals is described for application to the ISLs. In order to evaluate the CB noise of the ISLs, an emulation analytic system is established. The CB noise of two adjacent interorbital plane ISLs and two intraorbital plane ISLs of a low Earth orbit satellite in the constellation is analyzed. The system can also be applied to examine the CB noise of other ISLs. The simulation results conduce to the performance evaluation of the ISLs.