Heterodyne spectrometers

An overview is given on current heterodyne receiver technology and on heterodyne receivers for astronomical and atmospheric science applications. The principle of heterodyne receivers is explained. Technological developments in the fields of THz mixer, local oscillators (LOs) and wideband spectrometers are described. A review on existing airborne and spaceborne heterodyne receivers is given. Finally, possible future THz heterodyne receivers for the ESA-cornerstone FIRST (far-infrared and submillimeter telescope) and the astroplane SOFIA (Stratospheric Observatory for Infrared Astronomy) are outlined.     

Heterodyne spectra analysis

A complete explanation of heterodyne spectra is given here. The study of the folding effect has been carried out and the properties of heterodyne spectra are detailed. A fitting method using synthetic spectra is developed. Ozone heterodyne spectrum is taken as an example.     

Heterodyne schlieren system

A new heterodyne technique for the quantitative analysis of schlieren images is described. The technique is based on phase measurements of signals generated by a photodetector observing the variations in light intensity caused by a traveling grating of slits located at the knife-edge plane of a conventional schlieren system. The phase of the signal is proportional to the displacement of the slit image on the knife-edge plane as a result of the light deflection passing through a phase object. The displacement is proportional to the ray deflection angle and hence to the local gradient of the index of refraction. The technique, which is quantitatively precise and sensitive, is demonstrated by measurement of deflections caused by a lens with a focal length of f = 20 m. A displacement of 0.22 mm and a deflection of an angle of 5.2 x 10(-4) rad were detected.     

Heterodyne coincidence spectroscopy

A new type of light-scattering experiment, which should measure directly the triple static structure factor S ⁽³⁾ (k, q) of a fluid, is proposed. S ⁽³⁾(k, q) is the full spatial Fourier transform of the equilibrium triplet distribution function g ⁽³⁾(r ₁, r ₂, r ₃). The experiment may also be used to study dynamic correlation functions of the form <a_k (t)a_q (t′)a_k_q(t″)> (where a_k () is the kth spatial Fourier component of the density), thereby giving new information on mode-mode coupling. The method obtains its information from triple correlations in the arrival of scattered photons at three detectors. The detectors must be operated in the heterodyne mode (i.e. with a local oscillator); the scattering volume must be much larger than the volume over which molecular positions are correlated. Comparison is made with previous analyses of other multi-detector experiments.     

共路外差表面轮廓仪

本文提出了一种改进的外差共路干涉仪,用于表面粗糙度的测量,它对机械振动、温度变化等极不敏感,纵向分辨率为0.1nm,可用于表面粗糙度的实际检测工作.     

Heterodyne detection of enhanced backscatter

Heterodyne detection has been used to measure the enhanced backscattering from a standard target consisting of a plane mirror positioned behind a moving ground-glass disk (assumed to act as a phase screen). A tilt of the plane mirror in combination with spectral filtering of the detector output allows isolation of the double-scattered component of the light signal. When the illuminating laser and the local oscillator beams are correctly mode matched, the intensity of this signal displays the factor-of-2 increase that denotes full coherent enhancement. This full enhancement is preserved for a wide range of mirror to phase-screen spacing, in contrast with earlier observations in which direct-detection techniques were used.     

Heterodyne detection of synchrotron radiation

A time integral method for the study of resonant nuclear scattering of synchrotron radiation in the forward direction is presented. The method relies on the interference of radiation scattered by nuclei in two samples, one moving with respect to the other. The method, termed heterodyne detection of synchrotron radiation, gives the same information on hyperfine parameters as the well known differential method. The general formalism is developed for the case where the reference is a single line sample and the investigated sample has magnetic or quadrupole splitting. The first experiments are discussed. A comparison of time differential synchrotron radiation spectroscopy, heterodyne detection and Mössbauer spectroscopy is given.     

外差式偏振干涉成像光谱技术研究

提出了一种新型的基于Savart偏光镜的外差式偏振干涉成像光谱技术,该技术在偏振干涉成像光谱仪中引入一对平行偏振光栅对,使其得到的干涉图频率与波数相关,具有了波数外差的特点,降低了干涉图频率,从而可利用较少的采样点数实现很高的光谱分辨率.对外差式偏振干涉成像光谱技术的基本原理进行了研究,详细分析了系统光程差、干涉图表达式、光谱分辨率以及光谱复原方法等.最后给出了外差式偏振干涉成像光谱仪的设计实例并进行了计算机仿真模拟,所复原的光谱与输入光谱曲线相符合,验证了方案的正确性.外差式偏振干涉成像光谱仪具有结构紧凑、光通量高、稳定性强、光谱分辨率高的特点,尤其适合超小型高稳定性、高探测灵敏度的高光谱探测应用.     

Heterodyne beatings between frequency-shifted feedback lasers

Frequency-shifted feedback (FSF) lasers are potential candidates for long distance telemetry due to the appearance of beatings in the noise spectrum at the output of a homodyne interferometer: the frequencies of these beatings vary linearly with the path delay. In this Letter we demonstrate that these beatings also occur in the heterodyne mixing of two identical, but distinct, FSF lasers. This phenomenon is explained by the passive cavity model and is exploited to characterize the time-spectrum properties of FSF lasers. Consequences on telemetry with FSF lasers are presented.     

非对称空间外差光谱技术研究

传统空间外差光谱技术存在光谱分辨率、光谱范围与探测器象元数之间的制约关系。非对称空间外差光谱技术相比传统空间外差光谱技术主要区别在于增加单臂光栅到分束器的距离,能够在系统参数不变的情况下大大的增加光谱分辨率。首先阐述了非对称空间外差光谱技术的基本原理,并给出相应的系统参数计算公式推导结果,从理论上推导出单臂光栅偏置量增加和光谱分辨率增加之间的关系。偏置量作为非对称空间外差光谱技术的重要参数,受短双边象元数和光谱分辨率需求的制约。根据实验室现有实验平台参数,给出偏置量选择原则及结果。在元器件参数相同的情况下,分别计算了两种形式的理论光学性能参数,并且进行了仿真验证,得出非对称空间外差光谱仪与传统空间外差光谱仪光谱范围相同,但具有更高的光谱分辨率,并且分辨率提高与偏置量增加关系与理论计算相符。最后通过单色光扫描方法对非对称空间外差光谱仪实验室装置进行光谱范围和光谱分辨率的定标,定标结果与理论计算值吻合较好。     

光学外差探测信噪比研究

对于光学外差探测系统而言,为了有效利用回波信号,需要对信噪比这一表征系统性能的重要指标进行研究。从理论分析了信噪比,给出了计算信噪比的一般公式。理论分析表明,当探测器光敏面上本振光与信号光的振幅、相位、偏振都满足严格的匹配时,外差效率最大。对场分布为艾里函数的光外差信噪比进行研究和数值模拟。结果表明,当艾里斑尺寸的比例系数控制在0.8~1.2内,并使信号光斑主轴偏移量x0控制在0.5以下,探测器的尺寸在(0.6~0.8)λf/d内时,可获得大于0.7ηPs/hνB的信噪比。     

Heterodyne detection enhanced by quantum correlation

Heterodyne detectors as phase-insensitive(PI) devices have found important applications in precision measurements such as space-based gravitational-wave(GW) observation. However, the output signal of a PI heterodyne detector is supposed to suffer from signal-to-noise ratio(SNR) degradation due to image band vacuum and imperfect quantum efficiency. Here, we show that the SNR degradation can be overcome when the image band vacuum is quantum correlated with the input signal.We calculate the noise figure of the detector and prove the feasibility of heterodyne detection with enhanced noise performance through quantum correlation. This work should be of great interest to ongoing space-borne GW signal searching experiments.     

Heterodyne holographic microscopy of gold particles

We report experimental results on heterodyne holographic microscopy of subwavelength-size gold particles. The apparatus uses continuous green-laser illumination of the metal beads in a total internal reflection configuration for dark-field operation. Detection of the scattered light at the illumination wavelength on a charge-coupled-device array detector enables 3D localization of brownian particles in water.     

Heterodyne spectroscopy at very low frequencies

We have studied the spectrum of light scattered by capillary waves, thermally excited at the free surface of highly viscous liquids, by heterodyne spectroscopy techniques. We were able to measure spectral widths down to 0.2 Hz.     

瑞利型增强四波混频的外差探测

将光外差探测方法应用于瑞利型增强四波混频的研究,通过理论分析得到了瑞利型增强四波混频三阶极化率的表达式和外差拍信号的解析解,对拍信号的研究表明这种方法可以保留瑞利型增强四波混频极化过程的完整信息,获得瑞利型增强极化的吸收谱与色散谱.同时分析了三种不同的马尔可夫随机场模型下的差拍信号的区别与联系.     

Terahertz Heterodyne Imaging Part II: Instruments

This paper is Part II of a series of articles on Terahertz heterodyne imaging. In Part I introductory information and techniques were presented. In this paper the authors describe four different types of heterodyne imaging instruments that have been established at their facilities. In Part III (to appear in a future issue of this journal) selected applications of these instruments are discussed.     

Heterodyne mixing experiments with NbN-based SIS junctions

Tests of niobium-nitride-based SIS junctions in our experimental receiver at ENS¹ have shown the remarkable stability in time and through thermal cycling of these high T_C material junctions; they were used to evaluate the real junction temperature in our vacuum cryogenerator. Results of heterodyne mixing experiments are presented at 100 GHz, showing that further progress in fabrication would be necessary. Detection experiments were performed at 460 GHz with encouraging results.     

Heterodyne spectrometer for the 10 micron region

We first consider the interest of infrared heterodyne spectroscopy to study the vertical distribution of atmospheric constituents such O₃. Then an heterodyne spectrometer is described. It operates in the 10 micron region with a CO₂ laser and gives a resolution of 5 MHz. A laboratory spectrum of C₂H₄ is presented as a preliminary result.     

Multi-channel Heterodyne Radiometer on HT-7 Tokamak

The incoherent radiation emanating out of the tokamak plasmas gives vital information about the electron temperature. Electron cyclotron emission (ECE) is a powerful diagnostic tool for the measurement of electron temperature due to many advantages such as the high spatial and temporal resolutions. This paper presents the sixteen channel superheterodyne radiometer as an electron cyclotron emission diagnostic to measure the electron temperature on HT-7 tokamak.