Self-mixing interferometry based on a double-modulation technique for absolute distance measurement

A new, to the best of our knowledge, method for the measurement of the absolute distance of a remote target based on the laser diode self-mixing interferometry is presented. A double-modulation technique is introduced to improve the measurement resolution. Wavelength modulation of the laser beam is obtained by modulating the injection current of the laser diode. Phase modulation of the laser beam is obtained by an electro-optic crystal in the external cavity. Absolute distance of the external target is determined by the Fourier analysis method. Theoretical analysis and numerical simulations are given. Experimental results show that a resolution of +/-0.3 mm can be achieved for absolute distance ranging from 277 to 477 mm.     

Radio frequency controlled synthetic wavelength sweep for absolute distance measurement by optical interferometry

We present a new technique applied to the variable optical synthetic wavelength generation in optical interferometry. It consists of a chain of optical injection locking among three lasers: first a distributed-feedback laser is used as a master to injection lock an intensity-modulated laser that is directly modulated around 15 GHz by a radio frequency generator on a sideband. A second distributed-feedback laser is injection locked on another sideband of the intensity-modulated laser. The variable synthetic wavelength for absolute distance measurement is simply generated by sweeping the radio frequency over a range of several hundred megahertz, which corresponds to the locking range of the two slave lasers. In this condition, the uncertainty of the variable synthetic wavelength is equivalent to the radio frequency uncertainty. This latter has a relative accuracy of 10(-7) or better, resulting in a resolution of +/-25 microm for distances exceeding tens of meters. The radio frequency generator produces a linear frequency sweep of 1 ms duration (i.e., exactly equal to one absolute distance measurement acquisition time), with frequency steps of about 1 MHz. Finally, results of absolute distance measurements for ranges up to 10 m are presented.     

双光梳绝对距离测量实验研究

长度测量作为几何量测量关键共性基础技术之一,在科学研究、高端装备制造及空间探测等领域有着重要的应用。双光梳绝对测距技术充分发挥了光学频率梳的时域超短脉冲和频域高分辨特性,实现了高测量速率的绝对距离测量。目前双光梳测距存在测量范围小、测量精度低的问题,为解决此问题,提出了一种基于双光梳测距技术与相位测距技术相结合的方案,采用双光梳测距技术实现非模糊距离范围内长度的高精度测量,利用相位测距技术实现非模糊距离整数倍测量及指示功能,作为双光梳测距高精度测量的有效补充。搭建实验系统对该方案的可行性进行验证,结果表明在0~70 m的测量范围内,测量误差小于±5 μm。     

Multiplexing of optical fiber gas sensors with a frequency-modulated continuous-wave technique

We report on the use of a frequency-modulated continuous-wave technique for multiplexing optical fiber gas sensors. The sensor network is of a ladder topology and is interrogated by a tunable laser. The system performance in terms of detection sensitivity and cross talk between sensors was investigated and found to be limited by coherent mixing between signals from different channels. The system performance can be improved significantly by use of appropriate wavelength modulation-scanning coupled with low-pass filtering. Computer simulation shows that an array of 37 acetylene sensors with a detection accuracy of 2000 parts in 10(6) for each sensor may be realized. A two-sensor acetylene detection system was experimentally demonstrated that had a detection sensitivity of 165 parts in 10(6) for 2.5-cm gas cells (or a minimum detectable absorbance of 2.1 x 10(-4)) and a cross talk of -25 dB.     

Disturbance-free distributed Bragg reflector laser-diode interferometer with a double sinusoidal phase-modulating technique for measurement of absolute distance

A new range-finding technique that uses both double sinusoidal phase modulation and quasi-two-wavelength interferometry is described. Two independent interference signals are generated with respect to two different wavelengths on a time-sharing basis. We clarify that external disturbances of these interference signals are eliminated by both feedback control and differential detection and that the feedback control does not affect the distance measurement. A single distributed Bragg reflector laser diode allows us to simplify the optical setup and to improve the measurement accuracy. After discussing a measurement range, we estimate a measurement error by making several measurements.     

Accuracy of a multiple height-transfer interferometric technique for absolute distance metrology

A multiple height-transfer interferometric technique was developed to increase the absolute distance measurement capability of a metrology system that uses a tunable laser. Using multiple accurately calibrated reference heights, this technique relaxes the requirement of knowing accurate wavelength information for multiple wavelength interferometry while maintaining its advantages. We present an uncertainty analysis, analyze the primary sources of uncertainties limiting the performance of this technique, and discuss how errors can be minimized. Measurement results of 3D images obtained from a variety of objects are presented. The measurement uncertainty is experimentally demonstrated to be 0.3?μm over 50?mm for two discontinuous surfaces with a confidence level of 95% in a lab environment.     

Calibration standard for laser beam profilers: method for absolute accuracy measurement with a Fresnel diffraction test pattern

The absolute accuracy of a clip-level laser beam profiler is measured to the 0.3% level, by comparison of the profiler's reading to the known width of a Fresnel diffraction test pattern. A pair of opposed knife edges, illuminated by a quasi-uniform and quasi-plane wave, generates the pattern whose width is determined by the 50% cut points in translating the edge pair across a tightly focused beam. The convolution of the scanning aperture with the diffraction fringe pattern is modeled to remove the effect of the aperture size from the accuracy test and to give a means of measuring the aperture width. Discussions of the experimental aspects of this test method show it to be an acceptable calibration standard for optical profilers, of use to those working on the International Standards Organization draft standard for laser beam parameter measurements.     

Theoretical analysis of a noncollinear phase-matched optical parametric amplifier seeded by an optical parametric generation

We have theoretically analyzed the characteristics of an optical parametric amplifier system seeded by an optical parametric generation. We investigated the influences of the energy, pulse duration, material dispersion, and the third-order nonlinear effect in beta-barium borate. The group-velocity mismatch (GVM) becomes the most important factor for the amplification of bandwidths. Even though tilting the wave front of the pump can decrease the GVM, it seems impossible to generate pulses smaller than 10-fs with 400-nm pumping. However, 10-fs pulses can be achieved with a 30-fs pump duration with pumping at 800 nm.     

The highly sensitive optical measurement of absolute SAW amplitudes for power flow analysis

The established optical method for the measurement of surface acoustic wave (SAW) amplitudes (simple probing) has been modified to result in the effective separation of scattered light from the SAW-diffracted light by means of two single mode optical fibers. In this way, the photodetector noise is drastically reduced, and absolute SAW amplitudes of 10/sup -12/ in can be detected easily. Using an additional precise measurement of the nondiffracted light by means of fiber shifting, the SAW amplitudes can be calibrated with an error of less than 3%. This method can be applied to deliver an accurate power flow analysis in SAW devices. As an example, for a YZ-LiNbO/sub 3/ transducer, the measured and calculated SAW power densities were found to agree within better than 6%.     

Photoacoustic measurement of methane concentrations with a compact pulsed optical parametric oscillator

A pulsed periodically poled lithium niobate optical parametric oscillator operating in a cavity with a grazing-incidence grating configuration was used for sensitive and precise measurement of trace quantities of methane in nitrogen by photoacoustic spectroscopy with a novel differential photoacoustic detector. A sensitivity of 1.2 parts in 10(9) by volume of methane was obtained in direct calibration measurements (not extrapolated). With this apparatus, in situ measurement of the methane concentration in ambient air under atmospheric conditions was demonstrated.     

0.6328μmHe-Ne激光器小数重合法大尺寸绝对距离测量方法的研究

本文介绍了以小数重合法为基础的大尺寸绝对距离干涉测长的原理，并着重分析了两种测量方法：外差法和拍波法。     

A calibration-independent laser-induced incandescence technique for soot measurement by detecting absolute light intensity

Laser-induced incandescence (LII) has proved to be a useful diagnostic tool for spatially and temporally resolved measurement of particulate (soot) volume fraction and primary particle size in a wide range of applications, such as steady flames, flickering flames, and Diesel engine exhausts. We present a novel LII technique for the determination of soot volume fraction by measuring the absolute incandescence intensity, avoiding the need for ex situ calibration that typically uses a source of particles with known soot volume fraction. The technique developed in this study further extends the capabilities of existing LII for making practical quantitative measurements of soot. The spectral sensitivity of the detection system is determined by calibrating with an extended source of known radiance, and this sensitivity is then used to interpret the measured LII signals. Although it requires knowledge of the soot temperature, either from a numerical model of soot particle heating or experimentally determined by detecting LII signals at two different wavelengths, this technique offers a calibration-independent procedure for measuring soot volume fraction. Application of this technique to soot concentration measurements is demonstrated in a laminar diffusion flame.     

炸药爆速连续测量的杂波分析及新型探针的研制

针对商用连续电阻丝探针在实际爆速测量过程中易出现间断、跳跃等波形不连续问题,通过理论推导和数值仿真,分析了产生此类杂波的若干原因:外界电磁波干扰、探针中空气冲击波与金属射流干扰。同时结合分析结果,提出了消除各类干扰因素的方法,即通过元件外壳接地和对整个测试电路进行严格的电磁屏蔽;采用压致导通原理代替原先的高温电离导通,改进探针结构和制作工艺,以消除金属射流与降低前驱空气冲击波的影响,最终构建了抗杂波干扰的新型压导探针连续爆速测试系统,测得了较为光滑的爆速曲线,提高了测试数据的准确性。     

炸药爆速连续测量的杂波分析及新型探针的研制

针对商用连续电阻丝探针在实际爆速测量过程中易出现间断、跳跃等波形不连续问题,通过理论推导和数值仿真,分析了产生此类杂波的若干原因:外界电磁波干扰、探针中空气冲击波与金属射流干扰。同时结合分析结果,提出了消除各类干扰因素的方法,即通过元件外壳接地和对整个测试电路进行严格的电磁屏蔽;采用压致导通原理代替原先的高温电离导通,改进探针结构和制作工艺,以消除金属射流与降低前驱空气冲击波的影响,最终构建了抗杂波干扰的新型压导探针连续爆速测试系统,测得了较为光滑的爆速曲线,提高了测试数据的准确性。     

Fringe analysis method for electronic speckle pattern interferometry using only speckle patterns before and after deformation

Electronic speckle pattern interferometry is employed in many industrial fields as a useful deformation measurement method. However, two speckle patterns obtained before and after the deformation are necessary for measurement. Furthermore, at least three speckle patterns are required for high resolution measurement using ordinary fringe scanning technologies. In this paper, a novel method that can measure high speed deformations using a limited number of speckle patterns without using high speed cameras is proposed. The method enables application to dynamic deformation analysis because the method involves analysis using only two speckle patterns obtained before and after the deformation. A novel optical system that can record some spatial information into each speckle is set up for the method. In experimental results, it is confirmed that the out-of-plane deformation measurement can be precisely performed by the method and that the resolution power is almost equivalent to that of the ordinary method.     

Extra-cavity, widely tunable, continuous wave MgO-doped PPLN optical parametric oscillator pumped with a Nd:YVO4 laser

An extra-cavity, widely tunable, and continuous wave (CW) singly resonant optical parametric oscillator (SRO) based on MgO-doped periodically poled lithium niobate (PPMgLN) has been developed. A Nd:YVO₄ laser pumped by 808 nm laser diode array (LDA) is used as the pump source. The threshold value of the SRO system is only 1.86 W at 1064 nm. 1101.1 mW of idler output at 3.217 μm and 2004.3 mW of signal output at 1.590 μm have been achieved when the pump power is 8.76 W, and this corresponds to a total (idler + signal) optic–optic conversion efficiency of 35.5%. The periods of the domain structure on the PPMgLN wafer can be changed by shifting the PPMgLN crystal, thus enabling a widely tunable mid-infrared spectrum of 3.026–4.485 μm, and signal wavelengths widely tunable in range of 1.395–1.641 μm. Along with the signal and idler light, the visible and near-infrared spectrum (697.5–820.5 nm, 603.6–645.5 nm, 532 nm, 421.3–463.3 nm) is observed. This OPO system is compact, simple and operated at room temperature.     

Transient deformation measurement with electronic speckle pattern interferometry by use of a holographic optical element for spatial phase stepping

We have used a computer-generated holographic optical element (HOE) with electronic speckle pattern interferometry to calculate the interference phase corresponding to the deformation of a test object from a single TV frame. The HOE is a modified crossed phase grating that introduces a known phase change between the +/-1 diffracted orders, without being translated. The progressive propagation of transient mechanical waves was measured with an rms precision of 2pi/30.     

Absolute distance measurement with heterodyne optical feedback on a Yb:Er glass laser

Absolute distance measurement based on optical feedback using a single-frequency Yb:Er glass laser is demonstrated via the combination of heterodyne detection and frequency sweep. The technique allows for the enhancement of the sensitivity of the laser response to self-mixing thanks to resonant excitation close to the relaxation-oscillation frequency peak. The experimental results on noncooperative targets are in good agreement with the theory, and the shape of the resulting signal is analyzed in both the temporal and the frequency domains considering the specific dynamic of the class B solid-state laser. Suggestions are provided for further improvements on the signal processing.