调频光外差光纤位移传感器响应速度的研究

对调频光外差光纤位移传感器的动态响应速度进行了理论分析和实验论证。实验结果表明：调频光外差光纤位移传感器响应速度的提高，可通过增大调制频率和利用拍频信号的高次谐波成分来实现。     

光学法复现水声声压中的过零点解调系统设计

激光外差干涉是新一代水声声压基准的主要技术,光学干涉系统中的信号解调算法直接影响质点振速和声压量值。为准确得到测量结果,详细介绍了如何从多普勒信号中得到水质点振速的过零点解调算法,并建立了一套在线解调系统。该系统利用信号源产生两路相互正交的多普勒信号模拟光电二极管的输出,再经差分放大电路后进入示波器进行数据采集,最后由主机软件在线读取数据并完成质点振速解调。实验结果有效验证了过零点算法和解调系统的正确性和稳定性,所设计的过零点解调系统可直接应用于激光外差干涉法复现水声声压。     

Bragg光纤光栅法布里-珀罗传感器应变测量的研究

文章提出了一种对静态应变进行高分辨率测量的新方法。该方法是将 Bragg 光纤光栅(FBG)与全光纤法布里-珀罗传感器(FFPI)组合使用,利用 FBG 的反射波长漂移特性来测量干涉条纹的数目,利用 FFPI 和伪外差解调法对小数条纹进行解调,实现高分辨率的测量。通过实验测定,该系统对静态应变测量的分辨率达到了33nε。     

1 550 nm全光纤激光多普勒测振系统研制

针对目前国内激光测振系统价格昂贵、使用不便的问题,采用1 550 nm波段成熟的窄线宽光源和光纤元器件研制了一套低成本的全光纤激光测振系统原理样机。此原理样机光路部分采用马赫?泽德干涉仪结构,搭建了外差式激光干涉光路,参考光被40 MHz的声光调制器调制,与测量光在光电探测器表面发生干涉,产生原始的激光多普勒信号;信号解调部分采用相位解调法对原始激光多普勒信号进行解调,得到振动目标的运动特性,包括位移、速度和加速度信息。采用本单位的振动标准装置对其性能进行了测试,实验结果表明：在10 ~ 2 000 Hz的中低频振动范围内,1 550 nm全光纤激光多普勒测振系统峰值位移、速度和加速度的测量误差在-0.6% ~ 0.7%内。该系统在中低频段具有较高的测量准确度,且成本相对较低、操作便捷,具有技术借鉴价值。     

基于单片机的光纤光栅解调仪

光纤光栅的解调技术是光纤光栅传感器的关键技术,该文提出了基于单片机的光纤光栅解调系统模型,研究了可调谐F-P滤波法解调原理以及解调系统的框架.经实践证明,系统测量精度达到5pm,是低成本、通用的解调系统,是Bragg光栅传感器实用化的关键.     

基于干涉解调技术的光纤激光器水声传感系统

分析了基于Michelson干涉解调技术的光纤激光器水声传感的原理,在一段掺铒光纤中写入具有π相移的光纤光栅构成光纤激光器,水声压力作用在激光器上引起激光工作波长的变化;采用基于3×3耦合器的偏振无关的非平衡光纤Michelson干涉仪将激光波长变化转化为干涉仪的相位变化;干涉仪的输出由光电探测器转换后使用DSP进行信号解调.针对3×3耦合器分光比不对称的问题,本文提出利用实时调整幅度的2路干涉信号进行解调的方案,该方案不需要3×3耦合器有严格的分光比,消除了外界环境对解调输出的非线性影响.水声探测实验表明,光纤激光器水声传感系统的声压灵敏度为-166.5 dB(参考值1 rad/μPa),解调结果与水声信号具有良好的线性关系.     

Sagnac 干涉型水听器的信号解调技术

为了对Sagnac干涉型水听器的信号进行解调,在讨论了Sagnac干涉型水听器的原理,得到和Mach-Zehnder干涉型水听器的不同之处后,提出了相应的解调技术．并用软件实现了信号的解调．解调结果表明,输出信号与输出信号完全一致,并可以准确标定出干涉仪的相位调制．     

基于相位解调的光纤MEMS压力传感器

提出了一种新的压力传感器,该传感器基于Fabry-Perot腔干涉和相位解调理论测量压力。设计用MEMS技术以及普通的光通讯接插件制作出工艺简单,分辨率高的光纤MEMS压力传感器。阐述了传感器的工作原理,分析了硅膜的厚度对传感器性能的影响以及FP腔的长度对反射光信号的影响。研究了基于相位解调法的傅里叶变换方法,傅里叶解调法受光源强度波动的影响较小,解调精度高。实验结果表明改传感器在[0-3]MPa测量范围内,线性好,灵敏度可达到3.50 m/MPa（腔长改变/压力）。     

一种基于残周期拟合的激光测冲击信号解调方法

针对激光干涉法冲击加速度校准中调频信号的数字化解调问题,提出一种基于残周期正弦波拟合的解调方法,使用约1/5周期的残周期正弦波模型进行滑动拟合,实现FM信号的数字化解调,对激光多普勒冲击信号波形具有良好的适应性,无需信号的截取等操作,拥有良好的算法稳定性和一个采样间隔的时间分辨力。在一组实际测量波形上的实验,验证了所述方法的正确性与可行性,为冲击加速度的计算提供了另外一种有效实用的数据处理方式。上述方法也可用于外差式激光干涉振动信号的解调。     

基于小波变换的激光外差干涉微裂缝信号解调算法

为了解调出带有裂缝信息的微弱超声信号,提高激光外差干涉法探测材料表面裂缝的能力,研究了利用小波变换实现激光外差干涉微裂缝信号的相位解调算法,并将小波变换算法运用到实验获得的零差信号,提取出超声信号,得到了被测样件裂缝的位置信息.实验结果表明:利用小波变换算法得到的被测样件裂缝的距离与实际距离的误差率为2.40%,验证了算法的可行性和准确性.     

Demodulation of intensity and shot noise in the optical heterodyne detection of laser interferometers for gravitational waves

Demodulation of intensity noise in the optical heterodyne detector is analyzed for application in interferometric gravitational-wave detectors. The correlation function and the power spectral density of the demodulated intensity noise are derived, taking into account the effect of bandpass filtering at the photodiode and an arbitrary demodulation waveform. The analysis includes demodulation of the rf-modulated shot noise as a special case of the intensity noise. For shot-noise-limited detection, the signal-to-noise ratio is found as a function of the modulation parameters, and the optimization of the signal-to-noise ratio with respect to the demodulation phase is described.     

Characterization of chromatic dispersion of optical filters by high-stability real-time spectral interferometry

Chromatic dispersion of optical filters is characterized by what is believed to be novel broadband spectral interferometry, which is based on dual-wavelength heterodyne measurement of spectral phase. High phase stability is achieved by differential phase detection using two lasers for wavelength-swept probe and phase-tracking reference. The technique provides self-tracking interferometry by passive stabilization of optical phase and allows real-time measurement of spectral phase and group delay with a low phase drift of less than 0.04pi. A fiber Bragg grating and a thin-film filter are characterized by this method.     

Submicrosecond speed optical coherence tomography system design and analysis by use of acousto-optics

A novel high-speed no-moving-parts optical coherence tomography (OCT) system is introduced that acquires sample data at less than a microsecond per data point sampling rate. The basic principle of the proposed OCT system relies on use of an acousto-optic deflector. This OCT system has the attractive features of an acousto-optic scanning heterodyne interferometer coupled with an acousto-optic (AO) variable optical delay line operating in a reflective mode. Fundamentally, OCT systems use a broadband light source for high axial resolution inside the sample or living tissue under examination. Inherently, AO devices are Bragg-mode wavelength-sensitive elements. We identify that two beams generated by a Bragg cell naturally have unbalanced and inverse spectrums with respect to each other. This mismatch in spectrums in turn violates the ideal autocorrelation condition for a high signal-to-noise ratio broadband interferometric sensor such as OCT. We solve this fundamental limitation of Bragg cell use for OCT by deploying a new interferometric architecture where the two interfering beams have the same power spectral profile over the bandwidth of the broadband source. With the proposed AO based system, high (e.g., megahertz) intermediate frequency can be generated for low 1/f noise heterodyne detection. System issues such as resolution, number of axial scans, and delay-path selection time are addressed. Experiments described demonstrate our high-speed acousto-optically tuned OCT system where optical delay lines can be selected at submicrosecond speeds.     

Polarization mode dispersion measurement based on continuous polarization modulation

A novel technique is proposed for measurement of the group delay (GD) and the differential group delay (DGD) of optical material, components, and fiber. This new method is based on continuous polarization modulation of the stimulus optical field as opposed to sequential polarization state switching used in the traditional phase shift method. A new complete derivation of the phase shift method, based on the modified Jones calculus of elementary matrices, is presented. The derivation reveals that the phase shift measurement actually depends on all eight elementary parameters that represent DGD and optical frequency derivatives of polarization-dependent loss (PDL). Thus, the new expression for the phase shift includes the combined effect of PDL and DGD. The proposed method is evaluated by measuring a section of polarization-maintaining fiber and a 50 km length of single-mode fiber over a wavelength range from 1530 to 1610 nm. Measurements of DGD in a long single-mode fiber are shown to be highly insensitive to environmentally induced GD drift.     

Application of phase-to-amplitude conversion technique to linear birefringence measurements

A novel technique that measures the linear birefringence of crystal quartz within the configuration of a Soliel-Babinet compensator (SBC) is proposed. A characteristic of this technique is that phase retardation introduced by quartz is amplitude modulation (AM) instead of phase modulation (PM). The linear birefringence is measured regardless of the azimuth angle of the SBC and the orientation of the linear polarization laser beam. Compared with the single-wedge method, the SBC is similar to a parallel plate that allows for a wider range of refracttive index of the test material to be measured. This proposed method uses a conventional amplitude demodulation method in conjunction with an optical heterodyne technique and a bandpass filter to produce a better signal-to-noise ratio. Although the SBC configuration is more complex than a single element, the independence of azimuth angle and the orientation of the linear polarized laser beam can enhance the sensitivity of the linear birefringence measurement.     

Differential optical fiber refractometer based on a path-matching differential interferometer with temperature compensation

We present a new approach for the development of a highly stable optical fiber refractometer based on a path-matching differential interferometer. Exploiting a single-channel phase tracker and new synthetic heterodyne demodulations, one can eliminate the thermal drift on a piezoelectric transducer stack as a phase modulator by subtraction. A transducer in a differential Fabry-Perot refractometer is designed to compensate for the thermal effects not only from thermal expansion but also from the thermo-optic effect. The experimental data show that the refractive-index change in the sensing system can be kept at a level of approximately 5 x 10(-4) without serious variations for a 1-h period of long-term monitoring associated with a temperature variation of from 25 to 50 degrees C. Accordingly, the proposed new system can be easily implemented and used as a long-term monitoring system for medical care applications such as monitoring patients during drug injection.     

Circularly polarized optical heterodyne interferometer for optical activity measurement of a quartz crystal

Phase retardation between two orthogonal circularly polarized light waves that propagate in an optical active medium is proportional to its optical activity. The measurement of optical activity of a quartz depolarizer in terms of the phase difference of two orthogonal circularly polarized waves is proposed. A circularly polarized optical heterodyne interferometer with a Zeeman laser to measure the optical activity of a quartz crystal is demonstrated experimentally. The accuracy of the measurement is discussed. In addition, the effect of elliptical polarization and nonorthogonality of linearly polarized light waves of a Zeeman laser on the optical activity measurement is analyzed.     

Absolute interferometric distance measurement using a FM-demodulation technique

We propose an interferometric method for measuring absolute distances larger than the wavelength. A laser diode is used as a light source. The principle of operation is based on multiple-wavelength interferometry that uses a modulated light source. This method uses the fact that the wavelength of light emitted by the laser diode can be varied by means of the injection current. The modulation of the injection current in combination with the optical heterodyne technique causes a high-frequency phase-modulated detector signal. The phase deviation of the signal is a measure of the optical path difference in the interferometer. By FM demodulation of the detector output with a phase-locked loop demodulator, the optical path difference can be determined directly without the classical ambiguity problem of interferometry. The measuring range in the experiments was limited to 50 mm by the maximum travel range of the used specimen translation stage. Because of the inherent light sensitivity of the method described, the rangefinder can be used for three-dimensional profile measurements on a wide variety of objects, even on diffuse scattering surfaces.     

Recording invertebrate nerve activation with modulated light changes

Optical scattering techniques have the potential to provide noninvasive measurements of neural activity with good spatial and temporal resolution. We used the lobster nerve as a model system to investigate and record event-related optical signals with a modulated light source and heterodyne detection system. We observed changes in the transmitted birefringent light intensity, corresponding with electrophysiological measurements of the action potential. The photon delay was below the detection threshold, in part due to the small size of the nerve bundle. Our system allowed us to place an upper bound on the magnitude of the phase change of 0.01 degrees. The physiological stability of the preparation allows comprehensive characterization of biological and instrumentation noise sources for testing optical measurement systems.