Monitoring and multiplexing technique for interferometric fiber optic sensors with a linearly chirped Er:fiber laser

The monitoring of interferometer fiber optic sensors using a laser that is scanned over a wide frequency range is investigated. The interrogation technique is based on the principle that if the light-source frequency varies linearly with time, the optical signal reflected or transmitted is intensity modulated at a frequency that is proportional to the optical path difference (OPD) in the interferometer. Fourier components in the detected optical output signal then correspond to the OPDs of any interferometers that have contributed to this modulation. The temporal position of a peak in the power spectrum of this signal is proportional to the OPD of the interferometer that is responsible for that peak. A fine tuning of the OPD value is determined from the phase of the corresponding Fourier component. Experimentally, an Er:fiber laser scanned over a 48-nm range centered at 1540 nm was used to monitor intrinsic fiber Fabry-Perot interferometers (FFPIs). Variations in the laser scan rate were compensated with the optical signal modulated by a reference FFPI held at a constant temperature. The OPD measurement resolution was 3.6 nm, and the dynamic range was 1.3 x 10(7). The temperature was measured from 20 degrees C to 610 degrees C with a 0.02 degrees C resolution, and multiplexing of three of the sensors arranged in series was demonstrated.     

Electro-optic heterodyne interferometer

We propose a new configuration for using a triangle-wave signal to drive the electro-optic modulator in an electro-optic heterodyne interferometer system. The new configuration is adapted to measure the phase retardation of a wave plate and the optical rotation angle of a chiral medium. By adding optic elements, the second-harmonic component amplitude of the interferometer photodetector output signal became proportional to the phase retardation or optical rotation angle of the samples being tested.     

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.     

Experimental comparison of autodyne and heterodyne laser interferometry using an Nd:YVO₄ microchip laser

Using an Nd:YVO? microchip laser with a relaxation frequency in the megahertz range, we have experimentally compared a heterodyne interferometer based on a Michelson configuration with an autodyne interferometer based on the laser optical feedback imaging (LOFI) method regarding their signal-to-noise ratios. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity, while in the autodyne configuration, the wave beating takes place inside the laser cavity, and the relaxation oscillations of the laser intensity then play an important part. For a given laser output power, object under investigation, and detection noise level, we have determined the amplification gain of the LOFI interferometer compared to the heterodyne interferometer. LOFI interferometry is demonstrated to show higher performance than heterodyne interferometry for a wide range of laser powers and detection levels of noise. The experimental results are in good agreement with the theoretical predictions.     

Real-time tracking of time-varying velocity using a self-mixing laser diode

A new method is proposed for estimating the time-varying velocity of a moving target with a low-cost laser sensor using optical feedback interferometry. A new algorithm is proposed to track velocity variations from real-time analysis of the output signal of a self-mixing laser diode. This signal is strongly corrupted by a multiplicative noise caused by the speckle effect, which occurs very often with noncooperative targets used in many industrial applications. The proposed signal processing method is based on a second order adaptive linear predictor filter, which enables us to track the digital instantaneous Doppler frequency, which is proportional to the velocity. A model of the laser diode output signal is proposed, and it is shown that the sensor and its associated algorithm have a global first-order lowpass transfer function with a cutoff frequency expressed as a function of the speckle perturbations, the signal to noise ratio and the mean Doppler frequency. Numerical as well as experimental results illustrate the properties of this sensor.     

基于ACPT技术的纳米振动测量激光干涉仪

为实现高精度、高稳定的微小振动测量,设计了一种改进型迈克尔逊干涉仪,对其所采用的光学结构和解调算法进行了分析.信号解调方案基于载波调制和交流相位跟踪（ACPT）技术,相对应的光路采用双压电陶瓷（PZT）和准平面猫眼动镜以构成参考臂和测量臂,设计以改善信噪比、抑制低频干扰、增强系统稳定性、提高测量分辨力为目标.建立了纳米振动发生装置,搭建了振动信号测量系统,研究了激光干涉仪对振动信号的输出特性.实验结果表明,测量动态范围达到120 dB,信噪比高于40 dB,系统分辨力达到0.25 nm.该干涉仪可有效抑制诸如温度、湿度、压强、低频振动等带来的相位噪声的干扰,满足高精度、稳定、可靠的要求.     

Interferometric distributed optical-fiber sensor

An interferometric technique is described for detecting and locating perturbations along an optical fiber. This distributed sensor, based on a modified fiber-ring interferometer, has a position-dependent response to time-varying disturbances such as strain or temperature. These disturbances cause a phase shift that is detected and converted to spatial information. The sensor consists of two parts, namely, a reflecting-fiber-ring interferometer and a differentiating-ring interferometer. The reflecting ring consists of a fiber ring with one port of the coupler connected to a reflector. Consequently the output port of the reflecting-ring interferometer is the same as the input port. Because it is an inherent zero-path-imbalanced system, a short-coherence-length source such as a light-emitting diode can be used. Any time-varying perturbation on the fiber in the ring results in a detector signal proportional to the product of the rate-of-phase change caused by the perturbation and the distance of the perturbation relative to the center of the fiber ring. The second part of the system, a differentiating-ring interferometer, consists of the same fiber-ring interferometer modified only slightly. The output of this part of the sensor is proportional only to the rate of phase change as a result of the unknown perturbation and contains no distance information. By dividing the output of the reflecting-ring interferometer by the output of the differentiating-ring interferometer, we determine disturbance location. Results obtained with a 155-m distributed fiber sensor are discussed.     

Detection of slow geophysical perturbations in gravitational-wave interferometers

Techniques for measuring quasistatic geophysical perturbations with gravitational-wave laser interferometers are studied. The transfer function for this kind of apparatus with destructive interference (the dark spot regime) is calculated taking the natural asymmetry of the lengths of the arms of the Fabry–Perot optical cavities into account. Equations are derived for estimating the modulation in the power of the output signal at the photon circulation frequency in the arms of the interferometer owing to geophysical perturbations. These estimates are compared with observations on the LIGO interferometer.     

Interferometric antenna response for gravitational-wave detection

The interferometric antenna response to gravitational-wave excitation is studied with the Fermi Normal Coordinate (FNC) reference system, with the limit that the gravitational wavelength must be much larger than the interferometric arm length. An optical configuration of the antenna, quite similar to the one generally considered in the long-baseline antenna projects, has been studied, i.e., Fabry-Perot optical cavities in the interferometer arms and reflectors at the input and the output of the interferometer for the purpose of recycling both the laser power and the output signals. An exact computation of the antenna response is given in a form that is also suitable to provide directly the responses for simplier optical configurations without the power or the signal recycling. Furthermore the response of the antenna for the narrow-band detection mode is also calculated. The results obtained in the FNC gauge at rest with the antenna are consistent with the ones given in the literature that were computed in the transverse traceless gauge at rest with the gravitational radiation.     

In-line interferometer for direction-sensitive displacement measurements by optical feedback detection

We demonstrate a compact in-line interferometer for direction-sensitive displacement measurement by optical feedback detection with a semiconductor laser (SL) light source. Two reflected beams from a semitransparent reference mirror and a reflecting test object interfere in the SL medium, causing a variation in its output power. The reference mirror is located between the SL output facet and the test object. The performance of the interferometer is investigated numerically and experimentally to determine its optimal operating conditions. We have verified the operating conditions where the behavior of the SL output power profile could indicate accurately the displacement magnitude and direction of the moving test object. The profile behavior is robust against variations in optical feedback and scale of the interferometer configuration.     

Detection of Ultrasonic Signal Using Polarized Homodyne Interferometer with Avalanche Detector and Electrical Filter

Usually, ultrasonic detection is obtained by conventional contact techniques. In some critical cases such as working at hard environmental conditions (hot oven or nuclear field) or sensitive surfaces, then non-contact optical techniques should be used. However, for all optical techniques, low sensitive ultrasonic detection still a common problem. This paper presents a self building homodyne laser interferometer system which is modified by polarization state to improve the detection performance and to increase the sensitivity limit. The introduced interferometer system has been used to detect ultrasonic signal generated in coated quartz sample around its 10 MHz resonance frequency. He–Ne laser source has been used at 633 nm wavelength with low optical power of 1 μW. The generated ultrasonic signal has been observed by two different photodetectors to investigate the better performance one. Band pass electrical filter with certain specifications has been used to obtain suitable signal to noise ratio as demonstrated through this work.     

Comparative study of autodyne and heterodyne laser interferometry for imaging

For given laser output power, object under investigation, and photodiode noise level, we have theoretically compared the signal-to-noise ratios of a heterodyne scanning imager based on a Michelson interferometer and of an autodyne setup based on the laser optical feedback imaging (LOFI) technique. In both cases, the image is obtained point by point. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity (i.e., directly on the detector), while in the autodyne configuration, the wave beating takes place inside the laser cavity and therefore is indirectly detected. In the autodyne configuration, where the laser relaxation oscillations play a leading role, we have compared one-dimensional scans obtained by numerical simulations with different lasers' dynamical parameters. Finally, we have determined the best laser for LOFI applications and the experimental conditions for which the LOFI detection setup (autodyne interferometer) is competitive compared to a heterodyne interferometer.     

基于比例阀门的升降速度控制方法研究

提出了一种新的航空升降速度标准信号自动产生方法.从控制方面,采用硅压阻模拟、石英数字双传感器,分别作为过程控制传感器和基准压力传感器,保证系统升降速度的准确度.采用双闭环控制方案,模拟闭环采用电路设计实现,数字闭环通过计算机、数字传感器和软件实现.控制执行部件采用一对比例电磁阀门.控制电路设计采用了模拟电路PID和数字PID控制结合的串级控制方案,对实现升降速度和压力信号变化的微分电路进行了设计.实验证明,该装置升降速度的输出准确度优于0.2%F.S,控制范围为0～200 m/s.     

Interferometer for optical coupling and mode selection in a multichannel laser array

A theoretical consideration is presented of the optical coupling and selection of laser array modes by the use of a developed Lummer-Gehrcke interferometer as a resonator reflector. Control of the mirror reflection of the proposed interferometer permits laser power to be redistributed through channels on the outlet resonator mirror; in particular, it makes it possible to lead power out of the resonator by a single beam. In this way it is possible to diminish the sidelobes in the far-field radiation profile of multichannel lasers and to raise the efficiency of the optical coupling of the laser array with waveguides and fibers. This method may be used for the redistribution of laser power on the outlet mirror in striped lasers as well.     

Hybrid Wavelength-Time-Domain Interrogation System for Multiplexed Fiber Bragg Sensors Using a Strain-Tuned Erbium-Doped Fiber Laser

A system for the interrogation of fiber Bragg grating (FBG) sensors using a strain-tuned EDF laser with linear cavity is described. An optical switch is spliced to one end of the laser cavity and connects one of two high-strength draw-tower fiber Bragg gratings (DTGs). The gratings are simultaneously tuned by a stretching device and act as the end reflector of the laser cavity. By applying a ramp signal to the actuator synchronized to the optical switch, the laser signal sweeps over two different wavelength intervals, depending on the connected DTG. This approach represents a hybrid wavelength-time-domain interrogation for multiplexed sensors and doubles the number of sensors that may be addressed when compared with single DTG scanning. In addition, the use of the DTG allows a fivefold increase in the strain tuned wavelength interval over standard fiber Bragg gratings. An example application is demonstrated where temperature inside an electrical motor is measured during operation.     

Transparent network for hybrid multiplexing of fiber Bragg gratings and intensity-modulated fiber-optic sensors

A network for multiplexing fiber Bragg gratings (FBGs) and intensity-modulated fiber-optic sensors with no need to distinguish between the two kinds of sensor is proposed and experimentally demonstrated. Two FBG sensors and two intensity-modulated sensors are wavelength-division multiplexed; the electrical phase of the output signal is measured as a common parameter for both types of sensor. Furthermore, the intensity sensors become power referenced, and the FBG sensors are interrogated by a low-cost technique. Low cross talk is achieved by use of a tunable optical filter at the detector.     

Noncontact detection of ultrasonic waves using fiber optic Sagnac interferometer

This paper describes a fiber optic sensor suitable for noncontact detection of ultrasonic waves. This sensor is based on the fiber optic Sagnac interferometer, which has a path-matched configuration and does not require active stabilization. Quadrature phase bias between two interfering laser beams in the Sagnac loop is applied by controlling the birefringence using a fiber polarization controller. A stable quadrature phase bias can be confirmed by observing the interferometer output according to the change of phase bias. Additional signal processing is not needed for the detection of ultrasonic waves using the Sagnac interferometer. Ultrasonic oscillations produced by conventional ultrasonic piezoelectric transducers were successfully detected, and the performance of this interferometer was investigated by a power spectrum analysis of the output signal. Based on the validation of the fiber optic Sagnac interferometer, noncontact detection of laser-generated surface waves was performed. The configured Sagnac interferometer is very effective for the detection of small displacement with high frequency, such as ultrasonic waves used in conventional nondestructive testing (NDT)     

Synchronous phase detection for optical fiber interferometric sensors

A system has been developed to accurately detect phase signals produced in optical interferometric sensors. The system employs optical heterodyning and synchronously detects optical phase by feeding back an error signal to a phase modulator in the reference leg of the interferometer. This system is seen to have properties similar to a phase-locked loop. The system is mathematically analyzed and a simple second-order model developed which accurately predicts the system response.     

Embedded fiber-optic Fabry-Perot ultrasound sensor

A fiber-optic ultrasound sensor is presented. The sensor consists of a continuous length of single-mode optical fiber with a built-in Fabry-Perot interferometer. The acoustic pressure produces changes in the index of refraction along the interferometer cavity through the strain-optic effect, thus modulating the reflected power of the light propagating in the fiber. The dielectric internal mirrors that form the interferometer are fabricated by joining a fiber coating with a TiO(2) film at one end to an uncoated fiber by electric arc fusion splicing. Experimental results have been obtained for sensors embedded in plastic and graphite composite materials, using ultrasound waves in the range from 100 kHz to 5 MHz. Values for the optical phase shift amplitude as large as 0.5 rad were obtained at an acoustic frequency of 200 kHz for a 1.1-cm-long interferometer embedded in plastic.     

Self-mixing vibrometer with real-time digital signal elaboration

This work describes the development of an optical sensor for measurement of vibration without contact. The realized vibrometer is based on real-time digital elaboration of the signal obtained by a self-mixing interferometer, with an embedded autofocus system. Two different algorithms are implemented, for the continuous working on diffusive surfaces, with different levels of optical reflectivity. Thanks to the autofocus and the digital processing, the proposed sensor is easy to use and requires no assistance of a skilled operator.