Optical signal path delay fluctuations caused by atmospheric turbulence

We report the first direct measurements, to our knowledge, of optical signal path delay fluctuations caused by optical turbulence in the atmosphere. The experiments were based on satellite laser ranging. Our initial motivation was to identify all the random error contributors in satellite laser ranging. We measured and identified the random path fluctuations caused by the atmosphere in the range of units of picoseconds. An appropriate fluctuation model was developed.     

A laser feedback interferometer with an oscillating feedback mirror

A method is proposed to solve the problem of direction discrimination for laser feedback interferometers.By vibrating the feedback mirror with a small-amplitude and high-frequency sine wave,laser intensity is modulated accordingly.The modulation amplitude can be extracted using a phase sensitive detector(PSD).When the feedback mirror moves,the PSD output shows a quasi-sine waveform similar to a laser intensity interference fringe but with a phase difference of approximately ±π/2.If the movement direction of the feedback mirror changes,the phase difference sign reverses.Therefore,the laser feedback interferometer offers a potential application in displacement measurement with a resolution of 1/8 wavelength and in-time direction discrimination.Without using optical components such as polarization beam splitters and wave plates,the interferometer is very simple,easy to align,and less costly.     

Sinusoidal phase-modulating laser diode interferometer insensitive to intensity modulation for real-time displacement measurement with feedback control system

In sinusoidal phase modulating laser diode (LD) interferometer, the injection current of the LD is sinusoidally modulated to realize the modulation of the wavelength. However, the light intensity of LD is also modulated, which affects the measurement accuracy. An all-fiber sinusoidal phase modulating LD interferometer for real-time displacement measurement is proposed where the influence of the intensity modulation is eliminated with a new algorithm. It is made clear that an optimal depth of the sinusoidal phase modulation (SPM) exists in the algorithm. Moreover, the SPM depth is locked at the optimal value by controlling the injection current with a feedback control system. The feasibility of the proposed interferometer for displacement measurement is verified by experiments.     

Strong optical feedback in birefringent dual frequency laser

Strong optical feedback in a birefringent dual frequency He--Ne laser with a high reflectivity feedback mirror has been investigated for the first time. The output characteristics of two orthogonally polarized modes are demonstrated in two different optical feedback cases: one is for both modes being fed back and the other is for only one of the modes being fed back. Strong mode competition can be observed between the two modes with strong optical feedback. And when one mode's intensity is near its maximum, the other mode is nearly extinguished. When both modes are fed back into the laser cavity, the mode competition is stronger than when only one mode is fed back. The difference in initial intensity between the two orthogonally polarized modes plays an important role in the mode competition, which has been experimentally and theoretically demonstrated.     

Effect of atmospheric turbulence upon the spatial coherence of laser radiation

The present article is an analysis of the effect of atmospheric turbulence upon the spatial coherence of laser radiation. Young's experiment was used for an experimental study of the coherence of radiation traversing path lengths of 100 and 200 m in the atmosphere.The experimental measurements of the relation between the coherence function of the radiation and the temperature gradient are in strong agreement with the theoretical values. The relation between the coherence function and the distance between the field points considered disagrees with the theoretical values. The discrepancy can be explained by the limitations of the approximation of geometrical optics.     

多光束发射对湍流效应下激光传输的影响

大气湍流引起的光强起伏会导致大气闪烁、光束漂移、到达角起伏和波前畸变等效应.多光束发射技术是克服激光大气传输中光强起伏的有效途径之一.以单光束、双光束、四光束以及八光束发射为例,通过计算峰值光强、环围功率半径以及像散参数等光束质量参数,得到单光束和多光束大气传输的光强起伏特性.结果表明:随着光束数目的增加,峰值光强显著...     

激光雷达大气湍流测量方法研究

 报道了一种用激光雷达方法测量大气湍流的技术，利用发射激光束在空间所需位置产生一个聚焦光斑，通过此光斑处的大气分子瑞利散射机制形成一个空间的人造散射光源，接收系统对此光源的散射光进行探测，测量出从地面到此空间散射光源之间的大气湍流参数。给出了对武汉上空大气湍流测量的初步结果。得到了不同高度层的大气相干长度随时间的变化特性、平均值和起伏均方差，并对实验测量和模型计算的大气折射率结构常数在相应高度段的积分值进行了比较，二者积分值都在10-12量级，得到的结果基本相符。这种测量方法的实现，为测量空间不同方位、不同距离的大气湍流提供了一种新的手段。     

Generation of acoustic-gravity waves by atmospheric turbulence

Radiophysics and Quantum Electronics -     

Influence of non-kolmogorov turbulence on intensity fluctuations in laser satellite communication

Satellite laser communication holds the potential for high-bandwidth communication, but the atmosphere can significantly affect the capability of this type of communication systems for satellite-toground and ground-to-satellite data links to transfer information consistently and operate effectively. Usually the influence of atmosphere on satellite laser communication is investigated based on the Kolmogorov turbulence model. However, both increasing experimental evidence and theoretical investigations have shown that the Kolmogorov theory is sometimes incomplete to describe the atmospheric statistics properly, in particular, in some portions of the atmosphere. Considering a non-Kolmogorov turbulent power spectrum with power law −5 that describes the refractive-index fluctuations in the atmosphere above 6 km, we calculate the scintillation index of a lowest-order Gaussian-beam wave under the weak-fluctuation condition. Then, considering a combined power spectrum of refractiveindex fluctuations and using the expression obtained, we analyze the joint influence of the Kolmogorov turbulence from the ground to 6 km and non-Kolmogorov turbulence above 6 km on the scintillation indices of laser beams used in ground-to-satellite and satellite-to-ground laser communication links. We show that the scintillation index in satellite laser communication is equal to the sum of the scintillation indices induced by the Kolmogorov turbulence from ground to 6 km and that caused by the non-Kolmogorov turbulence above 6 km. Also we investigate variations of the scintillation index with the beam radius on the transmitter, wavelength, the radial distance, and zenith angle. Finally, comparing the scintillation index induced by these two turbulences with the conventional results, we show that the scintillation index induced by these two turbulences is a bit smaller than the conventional results.     

Diurnal variations in cosmic ray intensity caused by high latitude magnetospheric-ionospheric current systems

Cosmic ray intensity measurements obtained with the neutron monitors at Thule (geomagnetic latitude, 84.8°) and McMurdo (geomagnetic latitude, −79.9°) in 2007–2009 were used to test the effect of the magnetic field generated by magnetospheric currents flowing along geomagnetic field lines in the high latitude region on cosmic ray intensity. The existence of such a relation in a region where the geomagnetic field lines are virtually radial should revise our concept of the unimpeded access that cosmic ray particles have to the Earth’s atmosphere.     

基于大气湍流效应的双波长激光传输特性实验研究

为研究大气湍流作用下不同波长激光信号的传输特性,建立了基于双波长条件下的激光光束传输实验测试系统,在此基础上进行了室外实验测量,得到了双波长光束同信道传输时的光束漂移和光强起伏变化的测试数据。实验结果表明,大气湍流对激光信号的传输具有较大的影响,且光强的闪烁系数与波长有很大的关系,选择波长较长的激光束可减少湍流对光强起伏的影响;光束质心漂移具有很大的随机性,且其质心变化与波长之间无明显的直接联系。实验测试数据结果与大气湍流理论相符合,对空间大气通信具有一定的参考价值。     

Linewidth narrowing caused by optical feedback in a multi-mode vertical-cavity surface-emitting laser

We have studied experimentally spectral characteristics of a multi-mode vertical-cavity surface-emitting laser that is subject to optical feedback. Appropriate alignment of the feedback mirror can suppress higher-order modes and significantly decrease the spectral linewidth of the laser.     

Measurement of atmospheric turbulence strength by vortex beam

The behavior of spatial correlation singularities suggests a possible method for measuring atmospheric turbulence strength with a vortex beam. The refractive index structure constant can be obtained by measuring the radius of a ring dislocation of a vortex beam which has passed through atmospheric turbulence. An approximate analytic expression for the radius of a ring dislocation as a function of has been derived, and its accuracy is verified by numerical examples.     

Measuring the inner scale of atmospheric turbulence by correlation of lateral displacements of thin parallel laser beams

A method of determining the inner scale of atmospheric turbulence is described which is based on a property of the correlation functions of the displacements of thin parallel beams. This method is studied theoretically for the modified von Karman model of the spectrum of refractive index fluctuations, which has both inner and outer scales. Experimental applications of the method to both laboratory generated and outdoor turbulence are described.     

湍流大气中激光传输对傅里叶望远镜成像质量的影响

针对傅里叶望远镜系统激光在湍流大气中传输造成的成像质量下降,分析了湍流对光束传输特性的影响,指出成像质量的下降主要来自于上行传输链路中湍流造成的光束漂移与光束扩展,从而产生光束指向误差。分析了指向误差影响成像的机理。通过数值计算得出了不同强度湍流造成整条上行链路光束指向误差,并通过系统仿真,得到了不同强度大气湍流条件下的成像结果。结果显示:在弱湍流与中湍流条件下（大气折射率结构常数小于10-14 m-2/3）,随机指向误差较小(偏移比小于0.06),复原图像有较好的识别性;在强湍流条件下,成像质量下降严重。因此系统应选择避开强湍流地理位置与时段进行工作。     

Measurements of atmospheric turbulence strength by coherent optical scintillation

Summary The strength of the atmospheric turbulence is measured by an optical scintillation method. The obtained quantity is the structure constant at optical frequencies. As all turbulence parameters, this quantity is strongly affected by meteorological and local conditions, therefore it is of interest to measure it in different conditions and locations. The present measurements were made to characterize the turbulence at Florence. Paper presented at the 1° Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, Rome, June 19–22, 1984.     

Sinusoidal phase-modulating laser diode interferometer insensitive to the intensity modulation of the light source

In a conventional sinusoidal phase-modulating laser-diode (SPM-LD) interferometer, the wavelength of the LD is sinusoidally modulated by varying its injection current. However, the intensity modulation is associated with the wavelength modulation, which affects the measurement accuracy. We propose an SPM-LD interferometer insensitive to the intensity modulation of the light source, in which the influence of the intensity modulation is eliminated by choosing the appropriate sinusoidal phase modulation depth. Computer simulations and experiments are performed for real-time displacement measurement with the proposed SPM-LD interferometer. The measurement accuracy has been improved and the measurement repeatability is less than 1 nm. No additional components are required in our proposed method that leads to a simple system compared with the other previously proposed methods.