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.     

基于相位调制-解调的光纤激光相位噪声检测方法研究

在主动相位控制相干合成中,常用的光纤激光主动相位控制方法主要有外差法、多抖动法和随机并行梯度下降算法等. 基于多抖动法和外差法的原理,提出了一种新型的基于相位调制-解调的相位检测与控制方法. 该方法利用周期信号对参考光进行相位调制,将调制后的参考光与待检测信号光进行相干检测,通过对相干光强信号和相位调制信号的融合处理,实现相位噪声的检测和控制. 对该方法进行了数值模拟和实验研究. 实验结果表明,对于频率为2 kHz,噪声范围为[0,2π)的正弦噪声,相位检测精度优于λ/50,控制精度可达 关键词： 光纤激光 相干合成 相位噪声检测 调制-解调     

Instantaneous quadrature low-coherence interferometry with 3 x 3 fiber-optic couplers

We describe fiber-based quadrature low-coherence interferometers that exploit the inherent phase shifts of 3 x 3 and higher-order fiber-optic couplers. We present a framework based on conservation of energy to account for the interferometric shifts in 3 x 3 interferometers, and we demonstrate that the resulting interferometers provide the entire complex interferometric signal instantaneously in homodyne and heterodyne systems. In heterodyne detection we demonstrate the capability for extraction of the magnitude and sign of Doppler shifts from the complex data. In homodyne detection we show the detection of subwavelength sample motion. N x N (N > 2) low-coherence interferometer topologies will be useful in Doppler optical coherence tomography (OCT), optical coherence microscopy, Fourier-domain OCT, optical frequency domain reflectometry, and phase-referenced interferometry.     

基于快速扫描光学延迟线谐振扫描的相位调制方法

提出利用快速扫描光学延迟线(Rapid Scanning Optical Delay Line,RSOD)作为光学相干层析成像(Optical Coherence Tomography,OCT)系统的相位调制器,并采用正弦驱动方式(谐振扫描)来提高扫描频率,进而实现高频相位调制的方法.介绍了基于RSOD相位调制器的干涉信号包络解调方法,分析了大振镜低频率线性驱动、小振镜高频线性驱动和大振镜高速正弦驱动三种实施方案对OCT性能的影响,并针对谐振扫描方案中轴向扫描的非线性提出了图像畸变定量校正的方法.结果表明:RSOD大振镜谐振扫描的模式能够兼顾系统成像速度、成像范围以及系统信噪比的要求,是一种行之有效的相位调制方法.     

Full-field optical coherence tomography by two-dimensional heterodyne detection with a pair of CCD cameras

A two-dimensional heterodyne detection technique based on the frequency-synchronous detection method [Jpn. J. Appl. Phys. 39, 1194 (2000)] is demonstrated for full-field optical coherence tomography. This technique, which employs a pair of CCD cameras to detect the in-phase and quadrature components of the heterodyne signal simultaneously, offers the advantage of phase-drift suppression in interferometric measurement. Horizontal cross-sectional images are acquired at the rate of 100 frames/s in a single longitudinal scan, with a depth interval of 6 microm, making the rapid reconstruction of three-dimensional images possible.     

光学外差法测量Ba原子的自电离态

提出了把三光子共振非简并六波混频(NSWM)和双光子共振非简并四波混频(NFWM)应用于研究Ba原子自电离态的光学外差探测方法。采用该方法测量了Ba原子的6P_３／２19d自电离态,并观察到了三光子共振 NSWM和双光子共振NFWM信号干涉引起的信号增强。该方法是一种纯非线性光学方法,具有优秀的空间分辨率和简单的光路。其中双光子共振非简并四波混频信号作为本地振荡光束,三光子共振非简并六波混频信号作为信号光束,通过 改变信号光束的频率来达到用光学外差探测法研究Ba原子自电离态的目的。该方法中,实现相位匹配所要求的条件不是很严格,可以在很宽的频率范围内实现相位匹配,而一般的六波混频很难做到相位匹配, 并且所测量的信号是相干光, 当用窄带宽的激光时,可以获得消多普勒的分辨率。     

逆合成孔径成像激光雷达数据采样技术

逆合成孔径成像激光雷达能够实现对运动目标的高分辨实时成像,但激光信号的极大带宽和目标回波信号的微弱性给雷达回波数据的接收和处理带来了较大困难.针对这一问题,提出了基于光外差探测手段和压缩感知理论相结合的信号采样方法,首先通过光外差探测降低回波信号的有效带宽,再结合压缩感知理论实现对信号的稀疏化采样和重构.仿真结果证明了运用本文所提出的采样方法,在使用远低于奈奎斯特定理所规定的采样率时,仍然能够实现对目标的高质量成像.     

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。     

电荷耦合器件及其应用进展

电荷耦合器件（ＣＣＤ）是基于金属－氧化物－半导体（ＭＯＳ）技术的光敏元件。目前ＣＣＤ已具有光谱响应范围宽、检出限低、动态范围宽、暗电流和读出噪声低以及具有积分信号、多道同时检测信号和实时监测等能力的优点。目前它已经广泛地应用在各个领域。本文扼要介绍了ＣＣＤ的基本工作原理。特点及性能表征。评述了ＣＣＤ在光谱检测和成象领域中较活跃的应用及发展前景。指出了ＣＣＤ技术的发展给光谱分析领域带来了革命性的进展     

Heterodyne detection of multiply scattered monochromatic light with a multipixel detector

A new technique is presented for measuring the spectral broadening of light that has been multiply scattered from scatterers in motion. In our method the scattered light is detected by a heterodyne receiver that uses a CCD as a multipixel detector. We obtain the frequency spectrum of the scattered light by sweeping the heterodyne local oscillator frequency. Our detection scheme combines a high optical etendue (product of the surface by the detection solid angle) with an optimal detection of the scattered photons (shot noise). Using this technique, we measure, in vivo, the frequency spectrum of the light scattered through the breast of a female volunteer.     

微振动振幅的精密测量

本文给出一种微振动振幅的精密测量方法.此方法采用光拍相位检测、伺服锁定干涉仪光程差和窄带检测技术,大大提高了信噪比.微振动振幅检测灵敏度达2.2×10~(-2)(?)/Hz~(1/2)(2kHz信号).     

Frequency-multiplexed gas sensing using chirped laser molecular spectroscopy

A method for frequency-multiplexed multi-sample gas sensing is presented. It enables measuring multiple samples placed simultaneously in the setup, without any optical or mechanical switching. Samples are measured using heterodyne detection and signal from each sensing path is encoded at different carrier frequency. Subsequently, a signal from particular sample is retrieved through heterodyne beatnote demodulation at unique frequency. This technique is particularly suitable for real-time calibration of the sensor through a sequential (or simultaneous) detection of three signals: from unknown sample, reference sample and baseline. Basic setup is demonstrated and proof-of-concept experiments are presented. Very good agreement with spectra measured using standard tunable diode absorption spectroscopy is obtained.     

Heterodyne detection scheme for light-induced transient grating experiment

A novel heterodyne detection scheme for light-induced transient grating technique is presented. The heterodyne detection is achieved coherently mixing the picosecond pulses of diffracted and scattered light. A phase difference between these pulses is controlled by moving a holographic beam splitter along its grating vector. This approach is employed to study a competition of coexisting electronic and thermal optical nonlinearities in CVD grown diamond films in sub-nanosecond time domain and to determine carrier parameters.     

孔径透射率调制对外差探测的影响

信号光与本地光的夹角是影响探测器输出外差信号强度的主要因素。当夹角过大时,甚至不能检测到外差信号。通过分析提出,外差信号的振幅实际上是接收孔径透射率函数的傅里叶变换,以此为依据提出一种降低失配角对外差信号影响的方法,即在接收孔径处放置透射率分布满足一定函数形式的透过型元件,使其傅里叶频谱满足预定的要求,这种情况下两光束夹角增大时,探测器输出的外差信号强度能得到增强,从而降低了对探测系统瞄准精度的要求。     

调Q激光光折变自适应光外差探测技术研究

基于光折变晶体中自泵浦与互泵浦相位共轭共存效应,利用新型光折变自适应光外差探测系统进行了调Q脉冲激光的光折变自适应光外差探测技术研究.结果表明,该系统可以用来实现单纵模调Q脉冲激光的自适应光外差探测,并可以通过提高本振光强度有效地提高外差探测灵敏度.进一步的实验和理论分析表明,对于一般的调Q脉冲激光,由于其各个纵模频谱分量之间不相关,难于实现光外差探测.     

Optimal interferometer designs for optical coherence tomography

We introduce a family of power-conserving fiber-optic interferometer designs for low-coherence reflectometry that use optical circulators, unbalanced couplers, and (or) balanced heterodyne detection. Simple design equations for optimization of the signal-to-noise ratio of the interferometers are expressed in terms of relevant signal and noise sources and measurable system parameters. We use the equations to evaluate the expected performance of the new configurations compared with that of the standard Michelson interferometer that is commonly used in optical coherence tomography (OCT) systems. The analysis indicates that improved sensitivity is expected for all the new interferometer designs, compared with the sensitivity of the standard OCT interferometer, under high-speed imaging conditions.     

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。     

Multi-functional fibre-optic microwave links

The multi-functionality of microwave links based on remote heterodyne detection of signals from a dual-frequency laser transmitter is discussed and experimentally demonstrated in this paper. Typically, direct detection in conjunction with optical intensity modulation is used to implement fibre-optic microwave links. The resulting links are inherently transparent and mainly used for signal transmission. As opposed to direct detection links, remote heterodyne detection links can directly perform functionalities such as modulation, frequency conversion, and transparent signal recovery in addition to signal transmission. All three functionalities are presented and experimentally demonstrated with a remote heterodyne detection link based on a dual-frequency laser transmitter with two offset phase locked semiconductor lasers. In the modulating link a 1Gbit/s baseband signal is QPSK modulated onto a 9GHz RF carrier. The frequency converting link demonstrates up-conversion of a 100Mbit/s PSK signal from a 2GHz carrier to a 9GHz carrier with penalty-free transmission over 25km of optical fibre. Finally, the transparent signal recovering link transmits a standard FM video 7.6GHz radio-link signal over 25km of optical fibre without measurable distortion.     

基于受激辐射信号的谱域光学相干层析分子成像方法

鉴于不同生理病理状态下组织复折射率实部的变化不大,传统光学相干层析(OCT)成像技术在分子特异性识别上存在先天不足.为此,本文提出了基于受激辐射信号的OCT成像方法,可在实现传统散射成像的同时,实现基于受激辐射信号的分子成像.在超高分辨率谱域OCT系统的基础上,通过增设光谱分光与调制抽运光支路,建立了基于单宽谱光源的抽运探测谱域OCT系统,详细推导了调制抽运下受激辐射信号的获取与成像公式.利用搭建的抽运探测谱域OCT系统,实现了瞬态受激辐射信号的相干探测.基于同时获取的受激辐射OCT信号和传统OCT信号,成功重构了氮化物粉末构建样品的基于受激辐射信号的分子对比OCT图像.     

Calibration of high-frequency hydrophone up to 40 MHz by heterodyne interferometer

A calibration technique for high-frequency hydrophone utilizing a heterodyne interferometer is presented in this article. The calibration system is mainly composed of optical and signal processing modules. In the displacement measurement, a pellicle is mounted at the surface of water to avoid acousto-optical interaction. The phase modulated carrier signal is digitized and transferred to the computer, then processed by digital phase demodulation. A phase unwrapping algorithm is employed to remove ambiguity of the arctangent function and has proven effective in large displacement measurements. Pellicle displacement and voltage output of the hydrophone in focused ultrasonic field are processed by DFT to determine the amplitudes of the fundamental and harmonic components. Experiments show that the heterodyne technique can provide hydrophone calibration up to 40 MHz, with a slightly smaller sensitivity compared with the National Physical Laboratory (NPL) calibration results for most frequency ranges. Since the heterodyne technique is independent on assumptions about the geometry of the ultrasonic field and the performance of the transducer, it can be easily extended to high frequency and high power ultrasound measurement applications.