Squeezing the local oscillator does not improve signal-to-noise ratio in heterodyne laser radar

The signal-to-noise ratio for heterodyne laser radar with a coherent target-return beam and a squeezed local-oscillator beam is lower than that obtained using a coherent local oscillator, regardless of the method employed to combine the beams at the detector.     

Wavefront match simulation on receiver surfaces in coherent lidar

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Highly sensitive single-beam heterodyne coherent anti-Stokes Raman scattering

Single-beam coherent anti-Stokes Raman-scattering (CARS) microspectroscopy achieves a complete CARS scheme with a femtosecond laser. Here, we introduce heterodyne detection in a simple experimental extension: the optical fields driving the CARS process and the local oscillator used for heterodyning are derived from a single beam of ultrashort laser pulses by pulse shaping. The heterodyne signal is amplified by more than 3 orders of magnitude and is linearly dependent on the concentration of Raman scatterers. This dramatically increases the sensitivity of chemically selective detection at microscopic resolution while maintaining the simplicity of the single-beam setup.     

信号与本振光振幅分布对星间相干光通信系统混频效率的影响

星间相干光通信系统中,信号光与本振光的不同分布将影响外差混频效率.在假设信号光与本振光偏振一致、相位匹配、几何中心重合的前提下,详细推导了信号光振幅为平面波(Uniform)、高斯(Gauss)分布,本振光为艾里斑(Airy)、Uniform及Gauss分布时的6种外差混频效率关系式,其中4种得到了精确的解析式.在此基...     

部分相干双曲余弦-高斯光束的传输特性和空间整形

 通过加入高斯项的空间相干度，引入部分相干双曲余弦-高斯（ChG）光束，推导出部分相干ChG光束通过近轴ABCD光学系统传输的解析表达式，并用以研究其传输特性和空间整形。结果表明，部分相干ChG光束的光强分布不能保持传输不变性，其光强分布不仅与传输距离和离心参数有关，而且还与空间相干参数有关。在传输距离固定时，适当改变部分相干ChG 光束的离心参数和空间相干参数，可实现光束的空间整形。光束质量因子随空间相干性的增大和离心参数的减小而减小。高斯-谢尔模型光束和完全相干ChG光束可作为部分相干ChG光束的特例处理。     

基于调制光束的空间相干性获得局域空心光束

 通过调制部分相干光的空间相干性,实现了局域空心光束的产生。理论上根据范西泰特-策尼克定理及标量衍射积分,构造出具有一定空间相干性的部分相干光,并研究其聚焦特性。实验上采用理论上要求的光学系统来构造具有特定空间相干性的部分相干光,并采用薄透镜聚焦部分相干光。研究表明,当构造出的部分相干光经透镜聚焦,可得到局域空心光束。研究了光学系统的参数及透镜焦距对产生局域空心光束的影响,结果显示：光阑的拦截比及透镜的焦距越大,获得的空心光束的尺寸就越大。     

变光外差为电外差的双频激光探测

演示一种双频激光位移探测系统,阐述光载波激光雷达的概念。由单块非平面环形腔固体激光器和声光调制器产生100MHz载波频率的双频激光束,作为探测光束,经过光路收发系统,探测位于电动导轨上目标的位移变化,信号处理部分采用高速光电探测器响应后信号的电子外差解调方式,位移量的获得通过高频锁相放大器解算参考光束与信号光束的相位差并计算获得。通过双频激光把光学外差探测变为了电子外差探测,系统重复误差小于3%。系统在利用无线电雷达信号处理方式的同时,保留了激光探测的优点,位移测量系统具有良好的重复性。     

Propagation characteristics of partially coherent beams with spatially varying correlations

We introduce a class of partially coherent beams with spatially varying correlation properties. It is shown that mathematically simple modifications in the coherence function of conventional Gaussian Schell-model beams lead to partially coherent fields with extraordinary free-space propagation characteristics, such as locally sharpened and laterally shifted intensity maxima. We study the properties of such fields based on an elementary-mode interpretation and by numerical simulations. The results demonstrate the potential of coherence modulation for beam shaping applications.     

Polarization properties of polarized and partially coherent Electromagnetic Gaussian-Schell model pulse beams on slant path in turbulent atmosphere

This paper is based on the unified theory of coherence and polarization of stochastic electromagnetic beams and the extended Huygens–Fresnel principle, combined with the quadratic approximation of Rytov’s phase structure function and the generalized Stokes parameters. We have derived the novel expressions for the cross-spectral density matrix elements and the degree of cross-polarization of a class of elliptically polarized spatially and spectrally partially coherent Electromagnetic Gaussian-Schell model pulse (EGSMP) beams propagating through atmospheric turbulence along a slant path. Additionally, we calculate and analyze the effects of the turbulent intensity, the initial pulse duration, waist width of the beam, the spatial coherence length and temporal coherence length et al. on the polarization properties of fully polarized and partially coherent EGSMP beams. Finally, a comparison of the impact of those factors on the partially polarization beams is made. The results show that the influences of the turbulent intensity, the initial pulse duration, waist width of the beam, the spatial coherence length and temporal coherence length et al. on the polarization properties of fully polarized and partially coherent EGSMP beams are larger. While the effects of those parameters on the partially polarization and partially coherent EGSMP beams are smaller. It is noted that the results of this paper have established sound theoretical basis on the topic of improving performance of the laser system propagating through the atmospheric turbulence.     

A simple multiple beam lo coupler for SIS heterodyne arrays

The construction of SIS heterodyne imaging arrays for submillimetre wave-lengths requires multiple coupling of the local oscillator signal. The quasi-optical analogue of a multiple cross-guide coupler, employing 45° beam-splitters successively stacked along the local oscillator beam, allows for individual adjustment of local oscillator power to each channel. We analyse the coupling as a function of the focal ratio of the incident beams and the number of beam widths off-axis through which the coupler is extended, and describe a simple construction method to realise a compact and effective design.     

Coaxial combination of coherent laser beams

Based on polarization state conversion, a technique for coaxially coherent combination of laser beams is introduced. Laser beams can be coaxially coupled into one beam with high combination efficiency and perfect beam quality. A polarized laser beam combination system based on master oscillator power amplifier （MOPA） configuration is developed and the efficiencies of both unit combination and the whole system are investigated. In the experiment of combining four beams with single longitudinal mode, a combination efficiency of 85.3% is achieved. It can be further enhanced by improving the stability of experimental environment and the quality of optical and mechanical components.     

部分相干余弦-高斯光束

引入部分相干余弦-高斯光束,推导出部分相干余弦-高斯光束通过近轴ABCD光学系统的传输公式和M2因子,研究了部分相干余弦-高斯光束通过像散透镜的传输特性.结果表明,部分相干余弦-高斯光束的M2因子与部分相干余弦-高斯光束的空间相干参量和离心参量有关,但与透镜的像散系数无关.适当选取空间相干参量,离心参量或像散系数,在几何焦面处可得到类高斯、平顶和空心等不同的光强剖面,实现部分相干余弦-高斯光束的空间整形.     

激光相干阵列探测中的二元全息光栅

从外差阵列探测出发，围绕木振光束整形问题，就计算全息位相光栅在激光相干雷达系统中的应用进行了理论及实施方面的研究，得出较好的结果。     

Dual-signal heterodyne lock-in amplification with lasers

High-sensitivity heterodyne detection with lasers applied to radar and satellite communication is seriously hampered by the large electronic bandwidth due to Doppler shift and frequency instability. These drawbacks can be circumvented by dual-signal heterodyne detection. The system consists of mixing the local oscillator with two signal beams and the lock-in amplification at the beat frequency of the two intermediate frequencies. The signal modulation is demodulated by the lock-in amplification. A detailed analysis of the accompanying noise is given. The derived NEP is much smaller than that of a conventional heterodyne system. As examples, the system is applied to radar and space communication.     

Spatial coherence properties and directionality of partially coherent cosh-Gaussian beams

Based on the theory of optical coherence, the spatial coherence properties and directionality of partially coherent cosh-Gaussian (ChG) beams are studied. It is shown that unlike Gaussian Schell-model (GSM) beams, in the strict sense there do not exist partially coherent ChG beams which may produce the same far-field radiant intensity distribution as a fully coherent laser beam. However, under certain conditions it is possible to find partially coherent ChG beams with the same far-field radiant intensity distribution as a fully coherent laser beam.     

Quantum polarization fluctuations of partially coherent dark hollow beams in non-Kolmogorov turbulence atmosphere

Non-classical polarization properties of dark hollow beams propagating through non-Kolmogorov turbulence are studied. The analytic equation for the polarization degree of the quantization partially coherent dark hollow beams is obtained.It is found that the polarization fluctuations of the quantization partially coherent dark hollow beams are dependent on the turbulence factors and beam parameters with the detection photon numbers. Furthermore, an investigation of the changes in the on-axis propagation point and off-axis propagation point shows that the polarization degree of the quantization partially coherent dark hollow beams presents oscillation for a short propagation distance and gradually returns to zero for a sufficiently long distance.     

Studies of phase characteristics of various photon echoes with a phase sensitive detection method

Various optical coherent transient signals in ruby have been examined by using an optical phase sensitive detection (PSD) technique. The PSD is performed by comparing the phase of the heterodyne beat signals obtained by the Stark switching technique with that of a local oscillator whose frequency is equal to the heterodyne beat frequency. The optical FID, two-pulse photon echoes, radiation locked echoes and notched echoes have been detected as υ-mode signals with small u-mode signals probably due to the asymmetry of the inhomogeneous broadening. The dependence of the phase of the radiation locked and notched echoes upon the driving pulses is also examined. The experimental results are in good agreement with theoretical predictions based on Bloch equations.     

光纤激光相干合成中两种相位控制方法的比较

主振荡功率放大器(MOPA)相干合成是实现大数量光纤激光器相干合成的一条有效途径,而相位控制是其中的关键。对MOPA方案的两种相位控制方法——外差探测法和多抖动法进行了比较分析。并在此基础上,提出了一种新的相位控制方法。新方法增加了相干合成的光束数目,降低了实验操作难度。通过Matlab软件仿真表明,采用新方法进行64路光束相干合成时,相位均方根误差小于0.08λ,而使用多抖动法进行30路光束相干合成时,相位均方根误差为0.15λ。     

Characteristics of tilted and off-axis partially coherent beams reflected back by a cat-eye optical lens in atmospheric turbulence

In this paper, taking the tilted and off-axis partially coherent beams as the active detection laser beams, the characteristics of the active detection laser beams reflected back by a cat-eye optical lens in atmospheric turbulence are studied analytically and numerically. The analytical expressions for the centroid position deviation γ, the average intensity, the mean-squared beam width and the far-field divergence angle at the receiver plane are derived. It is found that tilted and off-axis active detection laser beams cannot be reflected back by a cat-eye optical lens in the direction of the source. The absolute deviation $\left| \gamma \right|$ decreases as the beam coherence decreases. Therefore, partially coherent beams are more suitable as active detection laser beams than fully coherent ones. In addition, $\left| \gamma \right|$ decreases greatly due to the aperture effect, and the influence of turbulence on $\left| \gamma \right|$ is not monotonic. On the other hand, the influence of the beam coherence and the atmospheric turbulence on the average intensity, the mean-squared beam width and the far-field divergence angle at the receiver plane is also examined, and some interesting results are obtained. The results obtained in this paper are very useful for applications of the active laser detection.     

扰动大气中部分相干光束的光谱特性

对扰动大气中传输的部分相干光束的光谱演化特性进行了研究。利用推广的惠更斯一菲涅耳原理．得到了扰动大气中部分相干光束的交叉光谱密度,据此分析了在大气扰动影响下部分相干光束的频移效应以及空间相干度的变化规律。结果表明：由于大气扰动的影响,在光束传输过程中,轴上光谱先是出现蓝移,蓝移量先增大后减小,传输足够远距离之后将出现红移,并且光束的相干性越差,就需要传播越远的距离才会发生红移;而光束的空间相干度随传输距离增大到一定程度之后开始减小,最终将趋于零。通过与自由空间中部分相干光的传输特性比较,发现当光束的传输距离比较小时．大气扰动效应对光谱特性的影响很小,只有在传输距离足够远时,大气扰动对光谱特性的影响才会明显地表现出来。