Sinusoidal phase-modulating laser diode interferometer insensitive to the intensity modulation of the light source

In a conventional sinusoidal phase-modulating laser-diode (SPM-LD) interferometer, the wavelength of the LD is sinusoidally modulated by varying its injection current. However, the intensity modulation is associated with the wavelength modulation, which affects the measurement accuracy. We propose an SPM-LD interferometer insensitive to the intensity modulation of the light source, in which the influence of the intensity modulation is eliminated by choosing the appropriate sinusoidal phase modulation depth. Computer simulations and experiments are performed for real-time displacement measurement with the proposed SPM-LD interferometer. The measurement accuracy has been improved and the measurement repeatability is less than 1 nm. No additional components are required in our proposed method that leads to a simple system compared with the other previously proposed methods.     

Sinusoidal phase-modulating laser diode interferometer for real-time surface profile measurement

A sinusoidal phase-modulating (SPM) laser diode (LD) interferometer for real-time surface profile measurement is proposed and its principle is analyzed. The phase signal of the surface profile is detected from the sinusoidal phase-modulating interference signal using a real-time phase detection circuit. For 60 × 60 measurement points of the surface profile, the measuring time is 10 ms. A root mean square (RMS) measurement repeatability of 3.93 nm is realized, and the measurement resolution reaches 0.19 nm.     

Large-range displacement measurement using sinusoidal phase-modulating laser diode interferometer

A signal processing method of realizing a large-range displacement measurement in a sinusoidal phasemodulating laser diode interferometer is proposed. The method of obtaining the dynamic value of the effective sinusoidal phase-modulating depth is detailed, and the residual amplitude modulation is also taken into account.Numerical simulations and experiments are carried out to compare this method with the traditional one.We prove that, with this method, the sinusoidal phase-modulating laser diode interferometer can realize a centimeter-level displacement measurement range with high precision, which is much better than the traditional method.     

Photothermal modulation of laser diode wavelength: application to sinusoidal phase-modulating interferometer for displacement measurements

In this paper, a novel laser-diode (LD) sinusoidal phase-modulating (SPM) interferometer, which utilizes a photothermal technique for LD wavelength modulation, is proposed to measure displacements with a nanometer accuracy. In conventional LD–SPM interferometers, the LD intensity modulation is concurrent with the wavelength modulation, which increases measurement errors. Using the photothermal technique, the LD wavelength modulation can be accomplished with negligible concomitant intensity modulation, and the measurement errors are thus eliminated. The computer simulations and experiment results verify the usefulness of this novel interferometer.     

A sinusoidal phase-modulating fiber-optic interferometer insensitive to the intensity change of the light source

In a conventional laser-diode sinusoidal phase-modulating (LD-SPM) interferometer, the wavelength of the LD is modulated by varying its injection current. The intensity modulation concurrent with the wavelength modulation leads to measurement errors. A photothermal-modulation method has been proposed to decrease the intensity change of the LD; however, this method cannot be used to measure vibration with a high frequency, and the beam diameter is too large to be used to measure minute objects. In this paper, we propose LD-SPM fiber-optic interferometer, in which the effect of the intensity change of the light source on measurement is eliminated. The diameter of the light beam is less than 0.5 mm. Using this interferometer, we measured displacements of a mirror driven by a piezoelectric transducer. The measurement repeatability is less than 1 nm.     

Real-time displacement measurement with large range and high accuracy using sinusoidal phase modulating laser diode interferometer

To resolve the conflict of large measurement range and high accuracy in the existing real-time displacement measurement laser diode (LD) interferometers, a novel real-time displacement measurement LD interferometry is proposed and its measurement principle is analyzed. By use of a new phase demodulation algorithm and a new phase compensation lgorithm of real-time phase unwrapping, the measurement accuracy is improved, and the measurement range is enlarged to a few wavelengths. In experiments, the peak-to-peak amplitude of the speaker vibration was 2361.7 nm, and the repeatability was 2.56 nm. The measurement time was less than 26μs.     

双频激光回馈位移测量研究

提出了一种基于双频激光回馈效应的位移测量系统，并在理论和实验两方面进行了研究.实验中对激光器输出功率变化进行了探测，同时对两正交偏振光的光强变化进行了探测.研究发现当被测物体移动时，激光器输出功率受到调制，物体每移动半个光波长时，回馈信号为调制深度相等的两峰，系统分辨率提高了一倍，达到四分之一光波长.当物体运动方向改变时，两正交偏振光的光强变化顺序也随之改变，可以实现物体位移方向的判断.理论分析及模拟结果与实验结果相符合.研究表明该位移传感系统具有结构简单、分辨率高、可判向、测量范围大、可工作于强回馈区等优点. 关键词： 激光回馈 位移传感 自混合干涉 双频激光器     

Polarization insensitive linear intensity modulation in a Sagnac fiber-optic loop with an electro-optic cell and optoelectronic feedback

We present a modulation scheme consisting of a fiber-optic Sagnac interferometer, which has an electro-optic modulator (Pockels cell) in the loop of the interferometer. In order to diminish the nonlinearity of the Sagnac interferometer transmittance function, we monitor a very small part of the output light intensity by means of a photodiode and then amplify this signal by a constant factor ξ. This amplified signal is added to the electrical modulating signal that is being applied to the electro-optical modulator. The consequence of introducing this feedback loop is that by properly choosing the amplifying factor ξ, we can increase the linearity of the modulation. The device is insensitive to the polarization state of the input light, so it should be used for intensity modulation of nonpolarized light with up to 100% modulation depth. The theoretical and experimental examination of the device is presented.     

A new laser displacement sensor based on triangulation for gauge real-time measurement

A new laser displacement sensor has been designed, constructed, and used in a train running at a speed of 64 km/h. The laser displacement sensor is insensitive to ambient light and to temperature. The parameters of the laser displacement sensor are as follows: the measurement range is 20 mm, and the laser displacement sensor resolution is 0.3 mm. All the results show that this new displacement sensor meets the requirement for real-time gauge measurement.     

具有位移和绝对距离测量能力的回馈干涉系统

提出一种基于半导体激光器泵浦Nd∶YAG微片激光器的回馈干涉系统，适合于非配合目标的位移和绝对距离测量。该测量系统包括移频回馈干涉光学系统，基于泵浦调制的微片激光器调频系统和相敏检波信号处理系统3部分。介绍了移频回馈干涉的原理，给出了位移测量和绝对距离测量应用的原理和测量方法，分析了测量的分辨率。实际测量了发黑工件表面的位移和绝对距离。实验结果表明，该回馈干涉系统灵敏度高，对被测表面反射率的要求很低，兼有对非配合目标的高分辨率位移测量和绝对距离测量的能力。     

Heterodyne interferometer for displacement measurement with amplitude quadrature and noise suppression

The high precision displacement measurement in nanoscale is crucial to many applications. We present a heterodyne interferometry with differential phase to amplitude conversion scheme for displacement measurement in nanoscale. In this approach, the differential phase introduced by the displacement is converted into the amplitudes of heterodyne signals in quadrature. Meanwhile, the heterodyne signals in phase quadrature are also achieved so that the displacement can be determined from the amplitude ratio of the quadrature signals, and the direction of displacement can be determined from the phase quadrature. Since the differential phase to quadrature amplitude conversion is achieved through the optical addition and subtraction by polarization tuning, which are based on differential detection concept. Thus the proposed method benefits from the features of differential detection with common phase noise and correlated amplitude noise rejection and that of quadrature detection with real time and wide dynamic range of phase measurement. To demonstrate the capability of proposed method in differential phase measurement, we measure the displacement drove by a commercially available PZT pusher and found close agreement between the experiment and the theory. The experimental evidence of noise suppression is also found with spectral measurements, which demonstrates the resolution of displacement measurement at 60 pm and minimum detectable differential phase of 5.6 × 10⁻⁶ rad/ over 50 kHz.     

基于单片机的LD控制系统的设计

为了实现激光器稳定、可靠和准确的功率输出,介绍一种基于单片机实现半导体激光器功率高稳定的控制系统。该系统以MSP430单片机为核心,根据半导体激光器的工作原理,设计了受控恒流源、温度控制系统和光功率反馈系统等部分。此外,系统还具有激光功率的实时控制、显示和设置以及软开关和软保护等功能。功率稳定采用光功率反馈法,温度控制采用高精度PWM驱动的半导体制冷器。光功率稳定度优于0.25%。     

基于双折射双频激光器中的调频回馈位移测量研究

提出了一种基于双折射双频激光器中的调频回馈效应的位移测量系统，并在实验和理论两方面进行了研究.实验中保持回馈外腔长度不变，将一四分之一波片置于激光器内腔中，调制内腔长度，激光器的中心频率发生改变，激光器输出强度将发生周期性的变化.研究发现激光强度曲线的波动频率正比于回馈外腔长度与内腔长度之比.当该位移测量系统的外腔长度是内腔长度的6倍时，被测物体每移动半个光波长，激光强度波动12次，系统分辨率可以达到二十四分之一光波长.理论分析与实验结果相符合.研究表明该位移测量系统具有高分辨率、大测量范围、可工作于任意回馈水平等特点. 关键词： 调频回馈 位移测量 双折射双频激光器 强度波动频率     

A microchip laser feedback interferometer with nanometer resolution and increased measurement speed based on phase meter

We introduce a new signal processing method based on phase meter into heterodyne microchip Nd:YAG laser feedback interferometer. The nanometer resolution and a higher measurement speed are realized. The factors determining the accuracy are analyzed. The displacements of the Physik Instrumente nanopositioning system and two piezoelectric transducers were measured. Experimental results indicate laser feedback interferometer’s ability of measuring nanoscale displacement and present promising application prospects in noncooperative targets measurement.     

Sinusoidal fringe projection system based on compact and non-mechanical scanning low-coherence Michelson interferometer for three-dimensional shape measurement

We report a sinusoidal fringe projection system based on superluminiscent diode (SLD) as a broad-band light source in conjunction with an acousto-optic tunable filter (AOTF) as frequency tuning device for three-dimensional shape measurement. The present system is based on a compact low-coherence Michelson interferometer system. The conventional interferometric system was modified in which one side of the beam splitter was coated with aluminum oxide which is used as reference mirror. With this modified version, interference fringes can easily be obtained by simply placing the external mirror in contact on the other side of beam splitter. Sinusoidal fringes with multiple spatial-carrier frequency can be generated in real-time using the present system by means of changing the radio-frequency signal to AOTF electronically without mechanically moving any component in the system. The present system was tested by projecting the sinusoidal fringes on a step-like object and 3D shape of the object was reconstructed using Fourier transform fringe analysis technique. The main advantages of the proposed system are completely non-mechanical scanning, easy to align, high stability because of its nearly common-path geometry and compactness.     

Phase perturbation investigation of laser diode for the laser diode to single-mode fiber coupling system

A novel phase-perturbation method as well as an experimental system were established to study the deviation of the laser diode (LD) far-field intensity distribution from the ideal Gaussian. An analytical expression of the LD far-field intensity distribution and a numerical method based on the Monte Carlo calculation were developed to fit the measured results. The loss of the LD to the single-mode fiber (SMF) coupling system caused by this kind of deviation of the LD far-field intensity distribution was quantitatively analyzed.     

High-performance laser power feedback control system for cold atom physics

A laser power feedback control system that features fast response,large-scale performance,low noise,and excellent stability is presented. Some essential points used for optimization are described. Primary optical lattice experiments are given as examples to show the performance of this system. With these performance characteristics,the power control system is useful for applications in cold atom physics and precision measurements.     

频率可调谐光栅反馈式扩展腔二极管激光器的搭建与测试

采用Littrow结构,用半导体激光器加上光栅反馈组成外腔可调谐半导体激光器.利用光栅的1级衍射光,反射至激光管选择激光频率,获得红光或近红外波长的单纵模、窄带宽、连续可调谐的激光输出.光栅平面同时作为激光外谐振腔的一个端面,外腔的模式选择、光栅选频作用,以及激光管本身的模式选择(内腔模式)的共同作用,最终产生可调谐的单纵模激光输出.     

High-precision measurement system based on laser interferometer for determining alcohol concentration of liquid solution

This study develops a high-precision, non-destructive measurement technique based on a laser interferometer for determining the alcohol concentration of a liquid solution from its refractive index. The optical metrology system is employed to measure the refractive indexes of samples with known alcohol concentrations ranging from 5% to 95%. By applying regressional analysis to the experimental results, an analytical expression is derived to describe the quadratic relationship between the refractive index and the alcohol concentration. An excellent agreement is observed between the experimentally determined values of the alcohol concentration and the analytically predicted results. For an assumed laser interferometer resolution of 1 nm, the measurement resolution of the proposed metrology system is found to be at least F = 0.0025%, which is significantly better than that of typical stereometry methods (approximately 1.0%) Furthermore, it is shown that the measurement resolution improves as the alcohol concentration reduces.     

Fiber interferometer for simultaneous multiwavelength phase measurement with a broadband femtosecond laser

We present a fiber interferometer for the simultaneous measurement of phase at multiple wavelengths from a single broadband femtosecond laser. Narrow-bandwidth fiber Bragg gratings isolate a particular frequency from the broad-bandwidth laser pulse produced. The multiwavelength phase data permit the unambiguous measurement range to be significantly increased compared with the wavelengths used in the interferometer. Preliminary experimental results are presented for a two-frequency sensor with an absolute range of 0.13 mm and associated dynamic range of 43,000:1.