Absolute distance measurement with an optical feedback interferometer

An important use of the self-mixing effect inside a frequency-modulated single-mode laser diode is in laser velocimetry and range-finding applications. The optical beam reflected by a target and injected into the laser diode cavity modulated by a reshaped current is mixed with the light inside the cavity, causing variations of the optical output power. A theoretical analysis of this effect is proposed, based on the determination of the beat frequencies of the optical power variations, to improve the accuracy of laser distance measurement. A resolution of ?1.5 mm from 50 cm to 2 m is obtained when thermal effects are taken into account.     

A low-cost laser range finder based on an FMCW-like method

The purpose of this paper is to present the prototype of a low-cost laser range finder. It belongs to the flight time measurement category. In this case, the time of flight is converted into a beat frequency proportional to the distance to be measured. This physical conversion is realized with an optical power modulation of the laser beam by a chirped wave. The experimental set-up is detailed with the main functional blocks. Experimental results are provided and the main possible improvements are discussed     

基于飞秒激光器光学频率梳的绝对距离测量

提出使用飞秒激光器的光学频率梳测量绝对距离的方法.将一个飞秒激光器作为绝对距离测量的光源,搭建迈克尔逊干涉结构,利用色散干涉原理进行相应的光谱分析,得到干涉光路的光学路径差引起的相位差,最终计算出干涉光路的光学路径差.实验结果表明我们的长度测量方法精确度高,分辨力达到纳米量级.最小测量距离达到9 μm,非模糊范围达到5...     

Review of low timing jitter mode-locked fiber lasers and applications in dual-comb absolute distance measurement

Passively mode-locked fiber lasers emit femtosecond pulse trains with excellent short-term stability. The quantum-limited timing jitter of a free running femtosecond erbium-doped fiber laser working at room temperature is considerably below one femtosecond at high Fourier frequency. The ultrashort pulse train with ultralow timing jitter enables absolute time-of-flight measurements based on a dual-comb implementation, which is typically composed of a pair of optical frequency combs generated by femtosecond lasers. Dead-zone-free absolute distance measurement with sub-micrometer precision and kHz update rate has been routinely achieved with a dual-comb configuration, which is promising for a number of precision manufacturing applications, from large step-structure measurements prevalent in microelectronic profilometry to three coordinate measurements in large-scale aerospace manufacturing and shipbuilding. In this paper, we first review the sub-femtosecond precision timing jitter characterization methods and approaches for ultralow timing jitter mode-locked fiber laser design. Then, we provide an overview of the state-of-the-art dual-comb absolute ranging technology in terms of working principles, experimental implementations, and measurement precisions. Finally, we discuss the impact of quantum-limited timing jitter on the dual-comb ranging precision at a high update rate. The route to highprecision dual-comb range finder design based on ultralow jitter femtosecond fiber lasers is proposed.     

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.     

飞秒激光频率梳测量光频用拍频装置的实验研究

飞秒激光频率梳测量光频实验中会遇到拍频信号信噪比低、高信噪比拍频信号维持时间短的问题。本文针对该问题在光路的准直、稳定维持方面采取了相应的光学、机械结构优化以及对拍频信号采取了先滤波后放大的措施;设计了一种拍频装置,并利用该装置进行了碘稳频633 nm激光器e峰频率的绝对测量实验。     

基于光路折叠的室内基线场误差分析方法研究

为了有效开展长距离光电测距类仪器的室内检定工作,在50m高精度导轨上利用1组平面反射镜进行光路折叠,搭建了100m室内基线场。对基线系统的测量误差进行了分析,考虑各项误差的调整精度,将误差分析结果应用到实际光路调整过程中,分析得到对基线长度影响更显著的误差量,并对其进行了控制和调整,提高测量光路的平行性调整精度,最后选择双频激光干涉仪作为长度基准开展验证实验。实验结果表明:通过光路折叠方法可以实现2倍光程倍增,基线系统的精度较高,可进行连续测量,有效地解决了室内基线建立过程中所存在的检测效率低、可重复性差等技术问题。     

采用法布里-珀罗腔的绝对距离干涉系统的方案分析

论述了线性调频绝对距离干涉测量方案的基本原理以及拍频测量中存在的问题;分析表明,在激光器调频范围确定的情况与,可以通过测量起点与终点拍频信号相位差的方法,实现１０＾－５的相对测量精度;简述了采用Ｆ－Ｐ腔实现高精度频率基准的方法,并给出了测量系统的原理图。     

Building blocks for a two-frequency laser lidar-radar: a preliminary study

A new principle of lidar-radar is theoretically and experimentally investigated. The proposed architecture is based on the use of an rf modulation of the emitted light beam and a direct detection of the backscattered intensity. Use of a radar-processing chain allows one to obtain range and Doppler measurements with the advantages of lidar spatial resolution. We calculate the maximum range of this device, taking into account different possible improvements. In particular, we show that use of a pulsed two-frequency laser and a spatially multimode optical preamplification of the backscattered light leads to calculated ranges larger than 20 km, including the possibility of both range and Doppler measurements. The building blocks of this lidar-radar are tested experimentally: The radar processing of an rf-modulated backscattered cw laser beam is demonstrated at 532 nm, illustrating the Doppler and identification capabilities of the system. In addition, signal-to-noise ratio improvement by optical pre-amplification is demonstrated at 1.06 microm. Finally, a two-frequency passively Q-switched Nd:YAG laser is developed. This laser then permits two-frequency pulses with tunable pulse duration (from 18 to 240 ns) and beat frequency (from 0 to 2.65 GHz) to be obtained.     

Distance and velocity measurements by the use of an orthogonal Michelson interferometer

A novel signal processing scheme for detecting distance and velocity signals simultaneously is demonstrated. In this method, a frequency-modulated diode laser is used to illuminate a dual-channel Michelson interferometer with two orthogonal output signals. The distance and the velocity signals then exist on the beat frequencies of the output interferometric signal. Two interferometric output signals with a quadrature phase shift are used to adjust the gating time period of frequency counters for beat-frequency measurement. The distance and velocity signals can thus be obtained from the counting number within the gated-in time period.     

绝对重力仪落体光心与质心空间距离测量

为了准确测量重力仪落体光心与质心距离,减少落体光心与质心不重合引起的旋转对重力加速度测量的影响,设计了落体光心与质心空间距离测量装置。提出了1种光心与质心间距的计算方法,并仿真验证了计算方法的可行性。使用设计的测量装置对落体进行测试,结果表明:激光干涉仪在信号频率为0.3~0.5Hz时的测量精度为0.1nm,提出的计算方法可以使测量系统的A类不确定度优于10μm,最大调校测量误差为1μm。     

Laser differential confocal radius measurement system

A laser differential confocal radius measurement system with high measurement accuracy is developed for optical manufacturing and metrology. The system uses the zero-crossing point of the differential confocal intensity curve to precisely identify the cat's-eye and confocal positions and uses an interferometer to measure the distance between these two positions, thereby achieving a high-precision measurement for the radius of curvature. The coaxial measuring optical path reduces the Abbe error, and the air-bearing slider reduces the motion error. The error analysis indicates the theoretical accuracy of the system is up to 2?ppm, and the experiment shows that the system has high focusing sensitivity and is little affected by environmental fluctuations; the measuring repeatability is between 4 and 12?ppm.     

Intersatellite range monitoring using optical interferometry

We report on an interferometer designed to provide 1-10 nm/square root(Hz) displacement measurement resolution, in the range 0.01 Hz to 1 Hz, while in low Earth orbit. The interferometer comprises two units, each with its own laser and in separate satellites, which would be in the same orbit separated by approximately 50 km. We discuss the requirements on the interferometer subsystem and describe the optical transponder distance measurement, including a phase locking method to generate a heterodyne beat signal between the two lasers. Design, fabrication, and testing of a "flightlike" engineering model interferometer is outlined, and results from environmental and performance tests are reported.     

Pyrometric measurement of the temperature and size of individual combusting fuel particles

A two-color pyrometric technique was developed for the in situ simultaneous measurement of individual fuel-particle temperatures and sizes in a pressurized entrained flow reactor (PEFR). A method that requires only a single optical port was developed, and a specially designed optical probe was manufactured for the measurements at the PEFR. The fuel-particle temperature was obtained by applying two-color pyrometry. The particle size was determined from the same pyrometric signals. A discrimination method was developed to confirm that the detected particle was valid for particle sizing. Several series of measurements were made at a PEFR at different process conditions, and some typical results are shown.     

Q-switched mode-locking of an erbium-doped fiber laser using cavity modulation frequency detuning

We present the results of an investigation regarding a Q-switched mode-locked fiber laser scheme based on a cavity modulation frequency detuning technique. The approach is based on undamped laser relaxation oscillations occurring due to frequency detuning in the fundamental cavity resonance frequency. Through a range of experiments with an erbium-doped, fiber-based, ring-cavity laser, this approach has been shown to be capable of generating high-quality Q-switched mode-locked pulses from an optical fiber-based laser. The maximum frequency detuning range for a stable Q-switched mode-locking operation has been observed to vary depending on the pump power used. We found that the highest pulse peak power was obtained at the frequency detuning threshold at which the operation changed from the mode-locking to the Q-switched mode-locking regime.     

Step-type optical delay line using silica-based planar light-wavecircuit (PLC) technology

We successfully fabricated a step-type optical delay line using a silica-based planar light-wave circuit (PLC) technology with a total delay of 51.0 mm and an optical coherent crosstalk of -49.1 dB. For the purpose of applying it to a mode-locked Er³⁺-doped fiber ring laser, we aimed for: 1) an optical delay of several tens of mm so that any optical pulse repetition frequency could be set, 2) as-low-as-possible optical crosstalk so that the oscillation of the mode-locked ring laser would be stable, and 3) low optical insertion loss. We integrated an optical circuit consisting of eight optical delay units possessing three thermo-optic switches, which have a Mach-Zehnder interferometer configuration, and two waveguides. The difference in optical path length of the waveguides corresponds to the optical delay. We set the optical delay of each unit to 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, and 25.6 mm, respectively, to give a maximum delay of 51.0 mm. Moreover, the total insertion loss was 5.2 dB. Precision measurement of the optical delay of each unit gave good results, with a manufacturing error of <0.47%, Based on the above results, the fabricated PLC step-type optical delay line can be used in a mode locked ring laser     

Real-time heterodyne imaging interferometry: focal-plane amplitude and phase demodulation using a three-phase correlation image sensor

A method of real-time heterodyne imaging interferometry using a three-phase correlation image sensor (3PCIS) is proposed. It simultaneously demodulates the amplitude and phase images of an incident interference pattern at an ordinary frame rate with good accuracy, thus overcoming the trade-off among measurement time, spatial resolution, and demodulation accuracy suffered in conventional interferometry. An experimental system is constructed with a 64x64 3PCIS camera operated at 30 frames/s and a two-frequency He-Ne laser with a beat frequency of 25 kHz. The results obtained for a scanning mirror and heated silicone oil confirm the proposed method.     

基于误差相消原理的光程倍增测量方法

针对大尺寸激光测距类仪器的量值溯源问题,研究了一种基于误差相消原理光程倍增的长距离测量方法,建立具有误差相消特性的测量模型,系统实现了4倍光程倍增,具有倍程结构简单、测量准确度高、测量稳定性好、连续测量等特点。实验复现了室内13m的虚拟长度基线,验证了测量方法的可行性,该方法有望将中远距离的基线场测距缩短至室内完成,将大空间的室内长距离测量缩短至小空间内的短距离测量。     

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.     

基于光纤延迟线的时间抖动测量系统

对飞秒激光器时间抖动的准确测量是推动其高精度应用的重要前提。为实现时间抖动的高精度、无参考测量,设计并搭建了一套基于光纤延迟线的测量系统。基于非对称迈克尔逊干涉仪的结构,在其中一臂引入一段长光纤延迟线鉴别时间误差,提高时间抖动测量精度。测量了一台实验室自制的重复频率为82 MHz的锁模光纤激光器的时间抖动功率谱,其在100 Hz~10 kHz的积分范围内RMS时间抖动为10.1 fs。