Twist sensor based on Sagnac single-mode optic fiber interferometer

An optical fiber twist sensor based on Sagnac single-mode optic fiber interferometer is proposed. The stress-induced birefringence of single-mode optical fiber is obtained by applying a transverse force against a short length of singlemode fiber. A high sensitivity and resolution of the twist angle measurement of 0.19 nm/° and 0.002° is achieved experimentally, respectively. The proposed sensor is more convenient and simple than that of standard polarization-maintaining fibers.     

光纤Sagnac干涉仪中单光子干涉及路由控制

介绍一种光纤中稳定的单光子干涉以及单光子路由控制方式. 使用Sagnac单光子环形干涉仪，通过分时相位调制，改变其顺时针和逆时针两路光子间的相位差. 在Sagnac单光子环形干涉仪中，顺时针和逆时针两路光子走过的是同一段光纤，简便有效地补偿了光纤长度随时间缓变带来的相位涨落，而且两路光子经历了相同的偏振模色散，较好地抑制了偏振态波动对单光子干涉的影响. 在长达5km的1550nm单模光纤中，获得大于98%的单光子干涉和大于90%单光子路由控制；在长度为27和50km光纤环路中，分别获得大于94%和84%单 关键词： 单光子路由 单光子干涉 Sagnac单光子环形干涉仪 量子保密通信     

Surface-bonded fiber optic Sagnac sensors for ultrasound detection

This paper describes a fiber optic sensor suitable for remote sensing and multi-point detection of ultrasound. This ultrasound sensor is based on the surface-bonded fiber optic Sagnac interferometer with the output fringe visibility of 1; it consists of a laser source, an ordinary single mode fiber delay line, a fiber coupler, a phase modulator and polarization controllers. For the validation of the sensor, surface acoustic waves and Lamb waves are excited by illuminating a steel specimen with an array of Q-switched Nd:YAG laser-generated line sources and the measurement of laser-generated ultrasonic waves are performed on the specimen surface using the surface-mounting fiber optic Sagnac sensor. The surface-bonded fiber optic sensor developed in this study has a simple configuration for detection of ultrasonic waves. Effectiveness of surface-bonded fiber optic Sagnac sensors for remote sensing of ultrasound and in situ monitoring of structures is investigated. The capability of multi-point detection of ultrasound by this Sagnac sensor is also discussed.     

分布式光纤Sagnac干涉仪中基于倒谱的多分辨率入侵定位算法

基于Sagnac结构的分布式光纤干涉仪具有灵敏度高和动态范围大等特点, 特别适合监测入侵信号, 并确定其发生位置. 不同于现有的时域自相关及频域陷波点定位方法, 本文提出将两相干光束相位差信号变换到倒谱域, 通过倒谱系数峰来方便准确地确定入侵位置. 此外, 为克服倒谱定位中降采样带来的分辨率粗糙问题, 提出了多分辨率定位综合的改进方法. 通过改变降采样因子和利用高阶倒谱峰两种措施对同一入侵信号得到一系列定位值, 并对所有定位值取平均作为入侵信号的最终定位值. 实验首先在距离传感光缆尾端40.498 km处制造3个入侵信号. 采用本文提出的倒谱方法对该入侵得到84个定位值, 综合后的定位误差分别只有9 m, 17 m 和 11 m, 显示了对入侵信号定位的准确性. 之后对不同位置的入侵信号进行定位, 7次实验的定位误差的标准差达到12 m, 而相同情况下的自相关方法和陷波点方法的定位误差标准差分别为695 m和118 m. 此外, 这种方法还具有操作简单、易于自动测量等特点, 有望在Sagnac干涉传感系统中发挥重要作用.     

Narrow band laser-generated surface acoustic waves using a formed source in the ablative regime

A narrow band laser-generated acoustic signal was created using a 4-element lenticular array. This arrangement of the array produces an acoustical signal with frequency content that is compatible with the response of a noncontact and remote broadband receiver, such as a capacitive air-coupled transducer. To support the experimental observations, a simplified concept is presented to explain the effect of a line array source on the frequency content of a surface acoustic wave. The analytical model solution for the wave front shape is derived from the point load solution of Lamb's problem that represents the displacement of a surface acoustic wave generated by an ablative line array. The distribution function, which was used for the model to represent the laser light energy, was tailored to depict the actual energy distribution that illuminates the lenticular array. Filtering functions are applied to the resultant surface displacement function to retain frequencies similar to those detected by the broadband 50 kHz-2.25 MHz receiver. The theoretical model showed good agreement with experimental results.     

Sagnac干涉仪与光纤环形激光器

依据相干光学理论,以现代激光干涉侧试技术中的光纤Ｓａｇｎａｃ干涉仪为例,分析和讨论了光纤环形激光器的基本工作原理,并结合其物理原理简要介绍了光纤环形激光器和光纤陀螺（ＦＯＧ）的应用的发展前景。     

Temperature measurements using fiber optic polarization interferometer

A novel measurement method of temperature based on the phenomena that the phase difference between principle polarization states in the optical retarder is function of temperature is described. The polarization state of optical beam is changed as it passes through the optical retarder, which depends on the temperature. The temperature of optical retarder is determined by comparison of the power difference between principal polarization states. We demonstrate successfully the temperature measurement by using a polarization maintaining fiber as the optical retarder. With a 100 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.236 rad/°C and the measurement error was ±0.038°C over the temperature range of −2.6 – +3.4°C. With a 11.5 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.021 rad/°C and the measurement error was ±0.79°C over the temperature range of −8.5 – +86.5°C.     

Performance of a wavelength-tunable erbium-doped fiber laser using a Sagnac interferometer

We experimentally demonstrate a wavelength-tunable erbium-doped fiber laser that is composed of a ring cavity and a single-mode fiber Sagnac interferometer in a new and simple arrangement. We find that the fiber laser output wavelength is tunable by adjusting the filter effect of the Sagnac fiber loop through a fiber polarization controller set there. The quasi-single-wavelength continuously tunable laser outputs could be achieved within some wavelength range. The multi-wavelength laser outputs could also be observed under some appropriate settings of the polarization controller. A theoretical demonstration of the wavelength tunability about the transmission-type Sagnac loop filter has also been achieved using the Jones calculus theory.     

Electro-optical fiber modulation in a Sagnac interferometer

The linear electro-optical effect was induced in optical fibers by thermal poling. A Sagnac all-spliced fiber loop was constructed incorporating an electro-optical fiber used as a phase modulator at 1.5 microm and stable switching was obtained. A rise time of approximately 40 ns was obtained, and signals <2 V generated a measurable optical response.     

Sagnac interferometer with adaptive nonlinear detection

We present a Sagnac interferometer that uses a nonlinear adaptive medium for detection. The nonlinearity of the medium being characterized by a finite response time, the detection has a finite frequency bandwidth determining two operational regimes: at low variation rate of the rotation speed the interferometer operates in the slow-light regime, providing a direct measurement of the angular acceleration. For high variation rate of the rotation speed the amplitude of the alternating component of the Sagnac phase shift is instead detected, with the continuous and low-frequency components filtered out by the adaptive nonlinear process in the medium.     

Rapid and accurate chromatic dispersion measurement of fiber using asymmetric Sagnac interferometer

We propose and experimentally demonstrate an advanced method for chromatic dispersion measurement of fiber. This technique is based on spectral interferometry by using an asymmetric Sagnac loop and broadband optical source. The chromatic dispersion can be directly obtained from the spectral interferogram seen from an optical spectral analyzer. This method is rapid (<1?s), accurate, simple, low cost, and can provide a large dispersion measurement range.     

Multi-wavelength fiber laser in single-longitudinal mode operation using a photonic crystal fiber Sagnac interferometer

In this paper, we report a new configuration of a tunable multi-wavelength erbium-doped fiber laser based on the commercial optical multiplexers and a Sagnac interferometer which includes a section of a high-birefringent photonic crystal fiber. Four-wavelength laser emission lines were obtained simultaneously in single-longitudinal mode operation showing a power instability lower than 1 dB, and an optical signal-to-noise ratio higher than 65 dB for all the emitted wavelengths. This proposed configuration allows the individual control of the loss of each channel of the laser and because of it is based on the commercial ITU-grid multiplexers, the laser is adapted to the telecommunications channel’s normative.     

激光激发瑞利波检测表面倾斜缺陷的研究

研究激光激发瑞利波检测样品的表面倾斜缺陷。基于频域热弹耦合方程,采用有限元方法建立激光激发瑞利波检测倾斜缺陷的数值模型,研究倾斜缺陷的检测机理。数值计算含不同的长度及倾斜角度缺陷的样品中瞬态位移波形,分析瑞利波在倾斜缺陷处模式转换的过程,研究各种瑞利波的传播路径。在此基础上,研究缺陷宽度与材料黏性对瑞利波传播及缺陷检测的影响。结果表明:瑞利波在缺陷处产生的反射及透射瑞利波的到达观测点的时间可以检测缺陷位置和长度,瑞利波在缺陷的底部发生模式转换产生的切变波可以检测缺陷倾斜角度。数值结果和已有的实验结果一致,从而为表面倾斜缺陷的检测提供有效的理论依据。     

Compact phase-shifted Sagnac interferometer for ultrasound detection

A compact fibre, phase-shifted Sagnac interferometer for ultrasound detection has been developed. The interferometer is a truly path-matched device, and therefore requires no path stabilization or heterodyning. It is a less expensive and more robust alternative to the heterodyne or path-stabilized Michelson interferometer. The device provides high spatial resolution of ultrasonic detection. It has been used in conjunction with conventional piezoelectric transducers (PZT) to detect Rayleigh and Lamb waves and to image a crack in a thin plate, rivet cracks in riveted plates, and for ultrasonic beam profiling.     

Phase-difference amplification using a Sagnac interferometer

In this paper, we describe a new method for achieving phase-difference amplification, which is quick and convenient, operates in real time, and requires no photographic steps. Magnification factors of 2, 4 or 6 are achieved easily in one step. Because the system operates in real time, phase stepping may be applied to extract the amplified phase distributions. Our method is a variation on longitudinally reversed shearing interferometry, using first- or higher-order diffraction from a grating (hologram) which is in fact the interferogram of the wavefront under test. The grating is derived from a standard two-beam interferometer which is phase-stepped, and displayed in real time on a spatial light modulator in the phase-difference amplification setup. It is illuminated by the two output beams from a Sagnac interferometer, similar to the set up used by (Barnes et al. Barnes TH, Eiju T, Matsuda K. Appl Opt 1986; 25:1864). for spectral resolution enhancement, and a phase-amplified fringe pattern is obtained by spatial filtering using a Fourier transform lens. We demonstrate operation of the phase amplifier and show amplified phase maps retrieved by phase-stepping. We believe this is the first time that real-time phase amplification without photographic steps and with phase stepping has been demonstrated.     

Unambiguous measurement of surface profile using a Sagnac interferometer with phase feedback

The fringe profiles from a two beam interferometer with phase feedback are almost unambiguous over a range of 2π or more. This allows phase and therefore surface profile to be determined directly from output intensity of an interferometer. In this paper we describe how feedback can be applied to a polarisation Sagnac common path interferometer using a parallel aligned nematic liquid crystal spatial light modulator. Initial experiments have shown that the device is capable of measuring profiles to accuracies greater than one tenth of a wavelength.     

光纤Sagnac温度传感器

介绍了一种新型的基于光纤Sagnac干涉仪的高精度光纤温度传感器———光纤Sagnac温度传感器。讨论了此传感器的测温原理,推导了被测温度与传感器输出之间的关系,设计并调测了一个具体传感器系统。实验结果表明,此温度传感器具有很高的精度和大的测量范围。     

Analysis of a Sagnac interferometer with low-birefringence twisted fiber

The fiber Sagnac interferometer of low birefringence and twist is analyzed numerically in the linear region. A novel method for measurement of the birefringence of the fiber and the angle of rotation of the axes inside the fiber loop of the interferometer is also presented.     

Hydrogen detection with surface acoustic transverse waves

Summary A new hydrogen sensor is here presented, based on the use of horizontally polarized surface transverse waves (STW). The use of these waves for application to chemical sensors seems attractive because of the possibility of controlling their penetration depth into the substrate and thus increasing the sensitivity of the device. Preliminary experiments, performed on a ST-cut quartz substrate, have shown how the response of the device strongly increases with the STW energy trapping in the sensitive Pd film.