Detection Schemes for Optical-Coherence-Domain Imaging of Biological Tissues

Optical coherence tomography (OCT) is a novel imaging modality based on low-coherence interferometry. This paper summarizes some of our latest work on the development of OCT where we have taken two approaches. In the first approach we have developed a phase-drift-suppression method for stable heterodyne detection in the presence of phase fluctuations. In the second approach we have concentrated on the development of two-dimensional heterodyne detection techniques for real-time imaging. A novel detection scheme has been proposed, which enables parallel heterodyne detection with a commercially available imager such as a charge-coupled-device camera. Meanwhile, a non-scanning OCT system configuration based on off-axis interferometry is being studied. Using a newly developed angular-dispersion imaging scheme, we show that the axial reflectance profile in OCT measurement can be detected instantaneously with a sensor array.     

Nonlinear chirp compensation by scan filtering in SAIL

The nonlinear chirp of a tunable laser generates the phase errors and damages the range resolution in synthetic aperture imaging ladar (SAIL). In the compensation algorithms establishing matched and nonmatched reference paths, the phase errors were compensated in a whole echo pulse. In this paper a compensation algorithm of nonlinear chirp by scan filtering is proposed.The heterodyne signals of different echoes from all target points in a footprint are scan filtered from one whole heterodyne signal of one whole echo pulse in the spectrum. The phase errors of these heterodyne signals are measured by phase-shifting algorithm in reference path and compensated separately. Then all the compensated signals are combined back into a whole heterodyne pulse and compressed in range. After all heterodyne pulses are compressed in range, the azimuth compensation is followed.The mathematical flow of this compensation algorithm is established. The simulation of the airborne SAIL model validates the feasibility, and the bandwidth of range compression decreases obviously. The effects of nonlinear chirp and the pass bandwidth of the scan filter are analyzed and discussed finally.     

Ultimate sensitivity of heterodyne spectrometers

Low noise heterodyne receivers are now used in a large variety of instruments such as radiotelescopes, far infrared laser side bands spectrometers, supersonic nozzle beams Fourier transform spectrometers, lidars or plasmas diagnostics devices. Numerous papers have been devoted to the analysis of heterodyne receivers in the attempt to reach ultimate performances. These previous papers usually retain the restrictive hypothesis of vanishingly small signal powers. Such a condition occurs only in a restrictive number of applications, radioastronomic systems being probably among the most representative examples. Spectroscopic instruments operate usually in a different regime. The signal to be detected consists of a small variation in the source power. The absolute value of this source power largely depends on the available technology in the spectral range of interest. The purpose of this paper is to analyze the behavior of heterodyne detection in the general case. The results apply to a large category of systems including spectroscopic instruments. The influence of crucial parameters such as source power and predetection bandwidth is analysed in details. The theory is experimentally tested on an example of ultra low noise receiver designed for far infrared laser side bands spectroscopy. Some numerical calculations are proposed, illustrating methods for an optimal sizing of receiver's predetection bandwidth and the estimation of ultimate sensitivity of spectrometers.     

全光纤双通道大气温室气体激光外差光谱探测技术研究

激光外差光谱技术是近年迅速发展的一种高光谱分辨率遥感探测技术,其装置具有体积小、光谱分辨率高等特点,适用于大气温室气体浓度的探测。各种观测实验已证明其是一种切实、有效的探测手段,在地球大气探测领域具有很大的应用前景和潜力。在现有激光外差光谱仪器的基础之上,提出了一种新型的仪器结构。采用光纤光开关对直射的太阳光进行调制与分束,实现利用单个光纤光开关进行两个波段激光外差信号的同时探测。为全光纤激光外差光谱仪的下一步系统集成和多波段激光外差光谱仪的构建提供了新方法。从激光外差探测的原理出发,分析了激光外差光谱探测技术的优势和关键参数,并结合自行研制的高精度太阳跟踪仪,搭建了一套近红外双通道全光纤式激光外差辐射计的原理样机。详细地阐述了激光外差辐射计原理样机中各功能模块的作用和参数,着重描述了光纤光开关在其中的作用原理和功能,详细地讨论了激光外差光谱仪的波长扫描模式和波长标定方法。在此基础上,讨论了激光外差辐射计的相关参数设置依据和仪器函数的测量方法,并给出了激光外差装置的仪器函数以及相应的光谱分辨率(0.004 4 cm-1)。利用搭建的激光外差光谱仪在合肥地区(31.9°N, 117.166°E)进行了实际的大气探测,同时在波段(6 056.2~6 058.1 cm-1)和(6 035.6~6 036.5 cm-1)分别获取了CH₄和CO₂分子的激光外差吸收信号,并对吸收信号进行了波长标定和归一化,得到了CH₄分子和CO₂分子的整层大气透过率谱,测量光谱信号的信噪比分别为197和209,同时也对分子吸收信号的光谱特征进行了分析。本文的测量实践表明光纤光开关可以用于激光外差光谱系统的结构优化,实现多通道多波段的激光外差光谱同时测量,拓展激光外差光谱仪在大气探测领域的应用。     

Remote sensing with infra-red heterodyne radiometers

The narrow spectral bandwidth capability of heterodyne radiometers can be utilized to construct highly selective pollutant sensing instruments. The infra-red spectral region is favourable due to the availability of pollutant absorption lines and the wavelength dependence of heterodyne sensitivity. We review the types of sensing systems which can make use of heterodyne techniques, including comments about the type of lasers which can serve as local oscillators. Sensitivity calculations of passive heterodyne radiometers to NO, CO₂, and SO₂ are then presented. Our concluding discussion involves the use of a heterodyne receiver in an active NO sensing system which is based on infra-red fluorescence.     

Pupil-plane speckle imaging with a referenced polarization technique

Pupil-plane speckle imaging is demonstrated by use of a polarimeter to estimate the complex wave front from an actively illuminated target. An experimental technique, referenced polarization imaging (RPI), closely parallels optical heterodyne imaging but is easier to perform in a laboratory. Further, RPI is less sensitive to target motions than the heterodyne method. The RPI technique is described, along with experimental results.     

CO2激光非线性四频外差探测的实验研究

本文从理论上对巳提出的四频外差探测技术作了阐述,并在实验上实现了非线性四频外差探测,得到了预期的结果.平方律放大器件是四频外差系统电信号处理的关键器件,本文对其原理作了叙述,并依此制成了一实用的器件用于本实验.     

Two-dimensional coherent detection imaging in multiple scattering media based on the directional resolution capability of the optical heterodyne method

This paper describes a new application of optical heterodyne detection using a laser beam for two-dimensional imaging of the internal structure of strongly scattering media in which the structure is completely obstructed from normal visual observation. The directional resolution capability for image formation due to the excellent antenna properties of the heterodyne technique is verified experimentally using a ground glass to cause strong scattering of the signal beam. Successful image detection of a test target placed in a highly scattering absorptive medium, with spatial resolution better than 400 m in the case of our experiments, demonstrates that this Coherent Detection Imaging (CDI) method can overcome the diffuse nature of images in media such as those of biomedical interest and others to achieve scanning and tomographic imaging.     

Photonic sensing technology is opening new frontiers in biophotonics

In advanced sensing photonics it is of great importance to explore the detection limits of extremely weak optical signals and imaging using state-of-the art technology. In this paper we describe recent progress in photonic sensing technology achieved in practice in the standard quantum limit of optical detection imposed by the signal-limited shot noise, which can be realized by both the optical heterodyne detection and photon counting techniques. Then, with particular attention on imaging of ultraweak photonic signals by these techniques, their applications in developing new frontiers in the field of biophotonics such as laser computed tomography, and the imaging and characterization of ultraweak biophoton emission phenomena are described and discussed as one of the typical examples of future trends in this field.     

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.     

像移补偿技术的发展与展望

航空相机广泛采用像移补偿技术来提高成像质量和相机分辨力。本文分析了像移补偿的原理并介绍了国内外较多采用的像移补偿技术,如光学式像移补偿法、机械式像移补偿法、集成像移补偿法、数字式像移补偿法等。文中列举了像移补偿过程中获取重要参数速高比的常用方法,包括平行狭缝法、扫描相关法、外差法、光程差法、直接计算法等。最后讨论了像移补偿技术数字化的发展方向,以及速高比测量的数字化、实时化、小型化发展趋势,提出了采用面阵CCD结合高性能微处理器获取速高比的新思路。     

多普勒振镜正弦调制多光束激光外差测量玻璃厚度的方法

本文提出了一种多普勒振镜正弦调制多光束激光外差测量玻璃厚度的新方法. 基于激光外差技术和多普勒效应, 通过做简谐振动的多普勒振镜对不同时刻入射光的频率进行正弦调制, 把待测厚度信息加载到外差信号的频率差中, 通过快速傅里叶变换对外差信号解调后可以同时得到多个待测玻璃厚度值, 经加权平均处理可以提高待测厚度的测量精度. 利用这种新方法, 通过MATLAB仿真测量了不同玻璃厚度值, 结果表明：该测量结果的最大相对误差小于0.008%.     

Doppler振镜正弦调制多光束激光外差二次谐波测量角度的方法

提出了一种多光束激光外差测微小角度的新方法. 基于激光外差技术和Doppler效应, 通过做简谐振动的Doppler振镜对不同时刻入射光的正弦调制, 把待测角度信息加载到外差信号的频率差中, 经信号解调后可以得到待测角度参数值, 经加权平均处理可以提高待测角度的测量精度. 利用这种新方法, 通过Matlab仿真了平面标准镜不同入射角的模拟结果, 表明该仿真结果的最大相对误差小于0.789677%. 关键词： 角度测量 多光束激光外差 激光Doppler技术 Fourier分析     

振动对潜望式光电仪器影响的数理分析

潜望式光电仪器具有细长桅杆的典型外形,对因桅杆振动所导致的瞄准线通过俯仰反射镜折转后在成像靶面上的移动量进行了分析计算,讨论了像点的移动对成像质量的影响,并提出了一些改善其观测效果的措施。     

一种新型外差光纤陀螺

本文提出了一种采用电光移频器和高双折射光纤构成的新型外差检测方式的光纤陀螺方案。它既能改善通常外差光纤陀螺中存在的光路非互易性且无需使用声光器件,又能避免采用高双折射光纤的直接检测光纤陀螺中电子线路直流漂移的影响。初步实验的结果显示出该系统具有良好的定标因子线性度。     

激光外差光谱仪的仪器线型函数研究

激光外差是一种基于相干探测原理的高灵敏度光谱检测技术,因其同时具有很高的光谱分辨能力,被广泛应用于诸多研究领域.在光谱测量过程中,仪器线型函数对吸收谱线的平滑作用,会对气体浓度的反演结果产生影响.为了获取激光外差光谱仪的仪器线型函数,基于激光外差原理和信号处理过程,对影响仪器线型函数的射频滤波带宽和积分时间等参数进行了分析,获得了仪器线型函数表达式.利用自行建立的激光外差光谱仪,多次测量了3.53μm波段内水汽、甲烷的吸收谱线,分别将射频滤波频域响应函数和本文获得的仪器线型函数耦合进水汽、甲烷柱浓度的反演.结果表明,射频滤波带宽为30 MHz、积分时间分别为10 ms和100 ms时,光谱仪的实际分辨率分别约为0.005 cm~(-1)和0.025 cm~(-1);使用仪器线型函数对积分时间为100 ms时测量的数据进行反演,透过率残差平方和与甲烷吸收峰值处的残差分别减小16%和100%,提高了气体浓度反演的准确度.     

像移补偿技术的发展与展望

航空相机广泛采用像移补偿技术来提高成像质量和相机分辨力。本文分析了像移补偿的原理并介绍了国内外较多采用的像移补偿技术,如光学式像移补偿法、机械式像移补偿法、集成像移补偿法、数字式像移补偿法等。文中列举了像移补偿过程中获取重要参数速高比的常用方法,包括平行狭缝法、扫描相关法、外差法、光程差法、直接计算法等。最后讨论了像移补偿技术数字化的发展方向,以及速高比测量的数字化、实时化、小型化发展趋势,提出了采用面阵CCD结合高性能微处理器获取速高比的新思路。     

大口径合成孔径激光成像雷达演示样机及其实验室验证

合成孔径激光成像雷达(SAIL)的主要技术指标如光学足趾尺寸和成像分辨率等都必须在所设计的远距离传输后实现.提出了一种在实验室空间近距离实现SAIL全系统贯通验证的方法,即在SAIL中附加光学装置使得其在近距离产生发射光束的准几何投影和足够大的外差接收视场,从而产生近距离二维SAIL成像必需的并且适当的光学足趾和方位向...     

混频技术测量单次脉冲微波频率的实验研究

 介绍了用混频技术测量单次脉冲微波频率的方法,用50ns、5GHz的单次脉冲微波对混频系统进行了实验研究。目前, 已用此技术测量了虚阴极振荡高功率微波锁频产生的单次微波的频率,并对混频输出的中频信号进行快速傅里叶变换(FFT),得到单次脉冲微波的3dB带宽。