光纤超声传感系统3×3耦合器输出信号的影响因素研究

光纤超声传感单位通常使用3×3耦合器作为核心干涉部件,其输出信号往往变化范围大、稳定性差,为了分析3×3耦合器输出信号产生不同现象的原因,开展了3×3耦合器输出信号的影响因素研究。应用干涉理论,分析了声信号和环境噪声对3×3耦合器输出信号稳定性的影响。数值计算结果与实验结果表明:声信号与环境噪声共同调制了干涉仪的相位,环境噪声使得3×3耦合器输出信号稳定性差;通过增大声信号幅值,使其引起的干涉仪相位幅值大于πrad,此时3×3耦合器输出信号的稳定性增强。     

可判向光纤位移干涉仪在振动测量中的应用

提出了基于光学多普勒效应和外差方法设计的可判向光纤位移干涉仪,装置采用光通信行业中已经发展成熟的器件,主要有带尾纤的半导体激光器、1×2光纤耦合器、三端口环形器、光纤探头、3×3光纤耦合器、探测器以及示波器等构成.在原有结构的基础上,增加了光纤放大器和光纤滤波器,大大提高了信号光光强.结合李萨如图形给出了可判向光纤位移干涉仪的信号处理方法.利用该干涉仪测量了压电陶瓷的振动,实验表明能够测量的最小振动峰峰值为0.43 μm,并根据实验分析了干涉仪测量微位移的一些制约因素.研究表明,制约干涉仪测量微位移能力的主要因素是3×3光纤耦合器的非理想性,如3×3光纤耦合器输出干涉信号的位相差不恒定.     

四级方向耦合器组成的全光纤波长交错滤波器

提出了一种由两个2×2和两个3×3光纤耦合器级联细成的全光纤Mach-Zehnder干涉仪型波长交错滤波器,由光纤传输理论,得到光谱的输出表达式.分析结果表明:组成滤波器干涉臂的第三对臂长差为第一对臂长差的2倍时,耦合器分光比取一些定值可得到输出波形通带平坦、透射率形态近似于方波的波长交错滤波器,谱线峰问的波长间隔约1.6 nm.通过实验所得的结果与理论相吻合.     

利用三个3×3耦合器串连构成干涉型三波长波分复用器

针对三通道波分复用技术,本文提出了一种利用三个3×3光纤耦合器级联细成的全光纤Mach-Zehnder干涉仪型三波分复用器,运用光纤传输理论和矩阵理论,得到光谱的输出表迭式.通过数值模拟分析了各3×3耦合器耦合比和光纤臂长差对该三波分复用器的影响,并进行了实验.结果表明,当组成的全光纤 Mach-Zehnder干涉仪型三波分复用器的三条光纤干涉臂两两之间分别存在长度差△L和2△L时,在耦合器分光比取一些定值时即可实现三波分复用功能,三端口输出谱线理想,波长间隔仅仅取决于△L的大小,且该器件的输出谱线对耦合器的耦合比不太敏感.实验样品采用的波段为1.55μm,波长复用间隔为0.8 nm,实验所得的结果与理论相吻合.     

利用WDM光纤耦合器的光纤光栅传感解调技术

根据 WDM 光纤耦合器波长解调方案的工作原理、偏振特性以及影响系统波长分辨力的因素,提出一种改进的利用 WDM 光纤耦合器的光纤光栅传感解调技术。该技术在原技术的基础上,采用偏振控制器控制入射光偏振状态,提高了解调的精度和稳定性。对 WDM 光纤耦合器的多次波长扫描结果表明,采用偏振控制器后,其波长误差可减小到 5pm 左右。实验采用 1540/1560nm的 WDM 光纤耦合器对单点光纤光栅应变传感器进行静态解调,结果表明:按此技术开发的解调系统具有 0.01nm 波长分辨力和 10nm 的波长线性解调范围。     

天津大学光纤传感技术研究部分最新进展

本文介绍了天津大学在光纤传感技术研究领域的最新进展.主要为:基于白光干涉实现了非本征光纤法珀和FBG并行解调,法珀腔长测量误差0.81 μm,FBG波长测量误差14 pm;基于光纤有源内腔结构夹现了乙炔气体传感,灵敏度优于100ppm;基于保偏光纤实现了分布式传感,灵敏度可达6 cm;基于边缘滤波器开发了光纤光栅解调仪,波长分辨力可达1.2pm,扫描速率超过200kHz;采用全光纤OCT技术实现了牙齿模型的二维、三维扫描;实现了光纤陀螺光纤环的温度、振动等动态特性检测.     

DFB光纤激光器声致弯曲振动研究

对利用DFB（分布反馈式）光纤激光器进行水声探测时的弯曲振动问题进行了分析与实验研究。总结了采用非平衡干涉仪解调系统解调的DFB光纤激光水听器的声压灵敏度计算公式;基于梁的弯曲理论,通过数值方法计算了两端固定的DFB光纤激光器在50Hz～2000Hz频率范围内的声压灵敏度,绘制了该频率范围内的频响曲线：采用振动液枉法对一支DFB光纤激光器在该频率范围内进行了实验研究,实验数据具有良好的可重复性,实验结果与理论分析吻合。表明了细长型结构的DFB光纤激光器在水声场中很容易由于弯曲振动而引入较大的非声压振动的干扰信号,影响其水声探测性能,有必要在DFB光纤激光水听器探头的设计中考虑这一因素。     

干涉型光纤传感器光正交外差解调技术研究

针对干涉型光纤传感器信号传感的原理特点,本文提出了一种光正交外差解调技术.该方法使用可调光纤延时线进行传感光信号延迟的精确调整以实现正交外差解调,从而减少了光电探测器的低频噪声影响.与常规的外差解调技术相比,提出的解调结构方案消除了电子移相元件所带来的额外电子噪声与环境温度变换引起的相位漂移.通过压电陶瓷环谐振特性测量实验,表明了提出的基于光正交外差的干涉型光纤传感器解调技术可以有效地进行干涉振动信号的解调,实验取得的最小可探测相移量可达到9.2× 10-5 rad.     

一种新型全光纤弹速测量系统的研制

研制了一种新型的利用激光光束反射原理的全光纤弹速测量系统。该系统采用了全光纤结构和光纤耦合器等无源器件,以输出光功率1mW, 工作波长1300nm的半导体激光器作为测试系统的光源,用光纤耦合器进行分光,实现了在一根光纤中同时传输光源和接受目标反射的信号光,避免了复杂的调节和准直过程。该系统结构简单、可靠性高,利用它,成功地测量了霍普金森杆发射的子弹速度,结果表明其速度测量相对不确定度小于1%。     

全保偏光纤直流磁场传感系统

采用保偏光纤耦合器和偏振器搭建了马赫-曾德尔型光纤干涉仪。在传感臂中接入铽镝铁材料被覆保偏光纤结构的光纤磁传感探头,在参考臂中接入保偏光纤PZT相位调制器进行工作点控制,得到了全保偏结构的光纤直流磁传感系统。该系统抑制了偏振诱导的信号衰落,实现了稳定的直流磁场信号检测。实验得到了系统对0.75mT~60mT的外加直流磁场响应特性,结果表明,该直流磁场传感系统的线性度达到了0.997,最小可检测的直流磁场达到3.4×10-6T。     

A Compact Fiber-Optic Flow Velocity Sensor Based on a Twin-Core Fiber Michelson Interferometer

A novel fiber-optic flow velocity sensor based on a twin-core fiber Michelson interferometer has been proposed and demonstrated. The sensor only is a segment of twin-core fiber acting as cylinder cantilever beam. The force exerted on the cylinder by the slow flow speeds of order mm/s of the fluid with unknown velocity bends the fiber, which corresponds to the shift of the phase of the twin-core in-fiber integrated Michelson interferometer.     

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.     

Technique for continuous tuning of optical fiber lasers

For the first time to the authors' knowledge, we have demonstrated how thermally controlled overcoupled fused fiber couplers and fiber loop mirrors based on these couplers can be used as broadband tuning elements in a fiber laser cavity. No bulk optical elements play any role in this technique. Temperature tuning the coupler results in a shift in the coupling ratio or in the effective output coupler transmission. For a fixed pump source, and for a given laser cavity, this shift causes the lasing wavelength to shift. We have continuously tuned an Er silica fiber laser in this manner over the range of 1527-1570 nm in a ring configuration, and, using a fiber loop mirror with these couplers in a linear Tm silica fiber laser cavity, we have achieved more than 50-nm broadband tuning over the range of 1850-1910 nm. The tuning range and the sensitivity to temperature depend on the degree of overcoupling of the loop mirror coupler.     

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)     

Dual Michelson interferometers for distributed vibration detection

A distributed fiber optic vibration sensor is described, in which two Michelson interferometers are used as phase detectors and two 3×3 couplers are deployed to demodulate the time-varying phase change caused by vibration. The two interferometers are separated by four wavelength division multiplexers. The position of the vibration is obtained by signal correlation, which can be used as a perimeter security sensor to locate the intruder. The experimental results with a 4012?m fiber sensor are discussed.     

External phase-modulation interferometry

We analyze and test a laboratory benchtop version of a compound interferometric phase sensor, a Michelson interferometer whose output is combined coherently with a phase-modulated local oscillator beam tapped off the Michelson input beam. This configuration models a whole class of external-modulation interferometers designed to shift signals, obscured by low-frequency intensity noise of the light source, into a shot-noise-limited region of the photocurrent spectrum. We find analytically that the shot-noise-limited sensitivity achievable with this system is comparable with that obtained by using internal phase modulation, with both schemes suffering (for different reasons) approximately a 22% sensitivity penalty compared with ideal shot-noise-limited direct detection. Experimentally we achieve true shot-noise-limited sensitivity, and we investigate trade-offs necessitated by commonly encountered nonideal features in any external-modulation system. Our analytic model, which specifically accounts for Michelson fringe contrast, electronic receiver noise, phase-modulation depth, and the local oscillator tap-off fraction, is sufficiently accurate to predict the absolute sensitivity of our benchtop instrument to within 0.5 dB.     

Passively mode-locked fiber laser sensor for acoustic pressure sensing

The development is reported of a multi-longitudinal mode fiber laser sensor based on passive mode locking employing carbon nanotubes in the laser cavity. A polymer membrane is employed beneath the pre-strained erbium-doped fiber (EDF) to convert the sound pressure disturbance into axial strain, alter the cavity length, and induce a shift of the longitudinal modes beat. Hence, acoustic pressure measurement can be carried out by detecting the shift of the beat frequency. Experimental results show comparable strain and sound pressure sensitivity of ~0.5 kHz/με and 147.2 Hz/Pa, respectively. The proposed sensor is an alternative for the measurement of acoustic pressure and possesses the advantages of good stability and ease of interrogation.     

SOA-based multi-wavelength source

A simple fiber laser configuration based on a semiconductor optical amplifier (SOA) is proposed for obtaining multi-wavelength oscillation at room temperature, in which a Sagnac loop mirror is used as the wavelength selective component. The SOA has a flat gain of approximately 23dB within a bandwidth of 12 nm at a small input signal power. The loop mirror was constructed using a 3dB coupler and polarization maintaining fiber (PMF). The output spectrum of the proposed laser can be adjusted by controlling the bias current of the SOA and is quite stable at room temperature. At a bias current of 150 mA, six lines are obtained with at least ?40 dBm output power and 25dB signal-to-noise ratio (SNR). The channel spacing and number of lines is determined by the length of polarization maintaining fiber (PMF) used in the loop mirror. The channel spacing of the proposed laser is 1.49 nm with a PMF 3 m. The multi-wavelength comb output can also be tuned by adjusting the operating temperature of the SOA. The multi-wavelength laser has the advantage of a simple configuration, stability at room temperature, a broad wavelength band, and no need for optical pump lasers.     

Side-hole two-core microstructured optical fiber for hydrostatic pressure sensing

A novel side-hole two-core microstructured optical fiber (STMOF) is proposed for hydrostatic pressure sensing. The two solid fiber cores are surrounded by a few small air holes and two large air holes, and are separated by one small air hole in the center of the cross section of the STMOF. The two large air holes that we called side holes essentially provide a built-in transducing mechanism to enhance the pressure-induced index change, which ensures the high sensitivity of the hydrostatic pressure sensor based on the STMOF. Mode coupling between the two fiber cores of the STMOF has been investigated, which provides a pressure-dependent transmission spectrum by injecting a broadband light into one fiber core of the STMOF on one side and detecting output spectrum on another fiber core on the other side. Our simulations show that there is a one-to-one correspondence between the hydrostatic pressure applied on the STMOF and the peak wavelength shift of the transmission spectrum. A hydrostatic pressure sensor based on an 8 cm STMOF has a sensitivity of 0.111 nm/Mpa for the measurement range from 0 Mpa to 200 Mpa. The performances of hydrostatic pressure sensors based on STMOFs with different structure parameters are presented.