Spectral-domain measurement of beat length in polarization-maintaining fibers

A spectral white-light interferometric (WLI) method for measuring the beat length in high birefringence polarization-maintaining fibers (PMFs) is presented. The approach is based on the stress-induced polarization mode coupling between the two polarization eigenmodes and utilizes their spectral interferograms. By recording the spectral interferograms before and after the displacement of the force, the beat length can be obtained by the displacement of the force and the relative phase variation retrieved from the two spectral interferograms. According to the principle that the phase difference between adjacent fringes peaks is 2π, the phase can be retrieved from the interferograms simply. The factors affecting the measurement precision of this method and the main source of errors are investigated in detail.     

Distributed PMD measurement with a polarization-OTDR in opticalfibers

This paper presents a new method for the measurement of distributed polarization mode dispersion (PMD) in optical fibers. This method uses a polarization optical time-domain reflectometer (P-OTDR), and is based on a measurement of the degree of polarization of the backscattered light as a function of distance in the fiber. Both the average and the statistics of the degree of polarization are used to estimate the two relevant parameters for measuring PMD, namely, the beat length and the coupling length. At present, our P-OTDR gives qualitative results only. However, it enables to distinguish between high and low PMD sections in a long fiber link. This should already have practical applications, in particular for the characterization of installed fibers     

Measurement of local beat length and differential group delay in installed single-mode fibers

We present beat length and polarization mode dispersion (PMD) measurements performed on installed fibers. Results regard three different kinds of fibers: standard step index, dispersion shifted and nonzero dispersion (NZD). After a historical comparison with standard differential group delay measurement collected four years ago on the same fibers, we perform a spatial-resolved measurement of the beat length by analyzing the state of polarization of the backscattered field. We compare PMD properties of different fibers and calculate the statistical distribution of the beat length. The differential group delay (DGD) and the beat length statistics depend strongly on fiber type and on fiber position along the link. The influence of the beat length on the DGD is also discussed.     

Distributed beat length measurement in single-mode optical fibersusing stimulated Brillouin-scattering and frequency-domain analysis

We present a method for distributed measurement of beat length, differential group delay, strain, and temperature in long length single-mode optical fibers. Toward this aim, we employ the polarization state sensitive effect of stimulated Brillouin scattering (SBS). The distributed measurement is realized by applying frequency-domain analysis. We present the analytical relationships between the Brillouin interaction of two counterpropagating waves in the fiber and the polarization states. Experimental results confirm the ability of the method to measure distributed beat length     

Influence of birefringence dispersion on a distributed stress sensor using birefringent optical fiber

Distributed stress sensor with a white-light scanning interferometer is used to detect stress distribution by analyzing polarization mode coupling caused by forces exerted on PMFs (polarization maintaining fibers). In measurement of polarization coupling, the birefringence in sensing fiber is usually considered to be wavelength independent. Sensitivity and spatial resolution of the distributed stress sensor are constant for a given optical source spectrum. In practical measurement, however, the birefringence in PMFs is related with optical wavelength. In other words, birefringence dispersion exists in PMFs. Due to birefringence dispersion, the relationship between stress and coupling strength varies with different positions of external forces, and spatial resolution of the distributed stress sensor descends obviously with transmitted distance. In this paper, influences of external-force positions and optical source spectrum on distributed stress sensor are analyzed in consideration of birefringence dispersion and validated with experiments. A method for compensating birefringence dispersion in the sensing system is proposed in the end.     

基于波长扫描调制法测量保偏光纤的偏振特性

分析了保偏光纤偏振特性及其拍长和偏振消光比的基本测量原理,并根据单模保偏光纤基模的2个正交偏振模分量HEx11与HEy11间的相位差与波长的关系,推导出偏振拍长的测量关系式,提出了测量保偏光纤偏振特性的波长扫描调制法.采用此方法对熊猫型保偏光纤(YOFC-3367WY)的偏振特性进行了验证性实验研究.拍长和偏振消光比的测量结果表明,在该保偏光纤工作波长处试验结果与理论分析相当一致,说明该方法具有简单易行、测量精度高的优点.     

光纤非线性偏振不稳定性的研究

光纤中的交叉相位调制将导致非线性双折射,从而引起偏振不稳定性。研究了光纤中线偏振光的入射角度与非线性双折射拍长的关系, 得出了入射偏振光接近快轴时,非线性使双折射增强,拍长变短的结论。这一结果与文献［２］ 的理论相反     

双轴光纤光栅在传感器上的应用

介绍分析了保偏光纤及双轴光栅的性质,阐述了双轴光栅用作传感器的基本原理,研究了它在三维应变测量、横向负载测量及双折射光纤拍长测量方面的应用．     

偏振保持光纤拍长的磁光调制法测量

叙述了利用磁光调制法测量偏振保持光纤拍长的基本原理，介绍了建立的测试系统和方法。     

双轴光纤光栅及其传感应用

简要介绍和分析了保偏光纤及双轴光栅的性质，研究了双轴光栅用作传感器的基本原理，介绍了它在三维应变测量、横向负载测量及双折射光纤拍长测量方面的应用。     

Optical-frequency encoder using polarization-maintaining fibre

A new method for monitoring the frequency modulation of coherent laser light is presented. The measurement system is an optical-frequency encoder which utilizes multireflection in a short polarization-maintaining fiber. The multireflection in each orthogonal polarization mode of the fiber produces beat signals by sweeping the optical frequency. The resultant two beat signals are monitored as an increase or decrease in optical frequency by holding the two signals in phase quadrature. The experimental setup, the principle for obtaining quadrature signals, and the results of monitoring distributed-feedback laser-diode frequency modulation are presented     

采用光干涉技术的保偏光纤拍长测量方法

简要介绍了采用光干涉技术的保偏光纤拍长测量方法,并给出了测量实例.与外调制测量法相比,该方法具有更高的测量重复性,测量原理也更为直观清晰.     

Photorefractive polarization couplers in elliptical core fibres

Photorefractive polarization couplers written internally in germanium-doped elliptical core fibers at 488, 514, and 532 nm are reported. Complete power transfer between the orthogonal polarization modes of the fiber was achieved for couplers written at 514 and 488 nm, respectively. It is shown that the couplers are nonuniform in length because of the high photoinduced attenuation and also due to two-photon absorption. Polarization coupling of higher order modes is also demonstrated at shorter wavelengths where their polarization beat lengths match the polarization beat length of the fundamental mode at which the coupler was written.<>     

一种双模光子晶体光纤模间干涉拍长的研究

保偏光纤的模间干涉拍长是模间干涉式光纤传感器设计中的关键参数,对于决定光纤的调制长度、传感系统的线性度和动态范围以及模间干涉灵敏度都具有重要作用。本文从理论上对保偏光子晶体光纤（PM-PCF）的模传输特性和模间干涉拍长进行了分析,得到PM-PCF中几个线性偏振模的传输特性和LP11偶模的两个正交偏振模截止波长,并得到了...     

保偏光纤及其技术发展趋势

本文介绍了保偏光纤的基本原理与制造方法,以及我国保偏光纤的技术进展状态,并阐述了保偏光纤的市场与应用领域,最后提出了保偏光纤前沿新技术方向。     

Measurement of the beat length in high-birefringent optical fiberby way of magnetooptic modulation

One of the most important parameters about high-birefringent optical fiber is the beat length. Because of its shortness and periodicity, the measurement of the beat length Is very critical. Here, a highly accurate measurement method that relies on magnetooptic modulation techniques is proposed, and the theoretical analysis and discussion about the measurement theories and the instrument construction are given in detail. Finally, the construction of the beat length measurement instrument based on this novel method is described and several kinds of high-birefringent optical fiber samples are measured. The beat-length resolution of this instrument is better than 0.01 mm, and the final measuring results are not influenced by the external interference on the fiber or the status of the placement of the optical fiber     

载波调制法提高保偏光纤偏振耦合检测的信噪比

保偏光纤偏振耦合系统能有效检测保偏光纤中偏振耦合点的空间位置及耦合强度,因而可被广泛地应用于应力、应变、温度和位置的分布式传感中。由于噪声影响,测试系统中的信噪比和耦合强度检测灵敏度会下降。为了改善系统信噪比,提高保偏光纤中弱耦合点的检测能力,将铌酸锂外调制技术应用于白光干涉保偏光纤耦合测试系统。调制后的信号经光电探测器接收,用数字解调算法进行数据处理。实验结果表明:在电机扫描速度为0.75 mm/s,调制频率为9.1 kHz,采样率为140 kHz时,载波调制时信噪比提高了8 dB,解调耗时仅1.12 s。     

Measurement of low-absorption coefficients using the beat frequency shift between transverse modes of a laser

A new method of absorption measurement of liquids is presented. This method takes advantage of the shift in beat frequency between a strong Gaussian mode, forming a thermal lens in the liquid, and a weak first transverse mode acting as a probe to detect the thermal lens. The light absorption coefficients of the liquids are directly related to the beat frequency changes that are measured on an RF spectrum analyzer. The approach is general in that it can be used to measure the focal length of any weak lens. It appears to be more sensitive by nearly an order of magnitude than the methods that measure change in spot size.     

光外差法测量光探测器频率响应的系统校准

通过实验证明了光外差法测量光探测器频率响应特性的一种校准方法的有效性.首先根据光外差法的基本原理详细推导了光探测器频率响应的表达式.发现了激光器调谐过程中的输出不稳定性是影响光外差法测量光探测器频率响应特性结果准确性的主要原因.激光器的输出不稳定性主要表现在输出光的偏振方向、光功率以及光谱不稳定造成的拍频信号线宽变化.实验对两个3dB带宽分别为10GHz和50GHz的探测器进行测量,逐一比较了采用与没有采用相应校准方法的结果,验证了该校准方法的有效性.     

Rate sensitivity of high birefringent fiber optic sensors underlarge dynamic loads

New experimental studies on the effects of large dynamic compressive loads on highly birefringent optical fibers are presented. The intensity rate of applied dynamic loads ranged between 2.18-6719 KN/m-sec. A Fibercore (previously York) HB600 Bow-Tie fiber with an initial beat length of 1.16 mm was employed in the present study. Sensor response is discussed in terms of the effects of force amplitudes and velocities on the calibration parameter T_f. T_f is a function of the amplitude as well as the velocity of applied loading. Therefore, calibration of the sensor should be achieved through regression analysis of the load-fringe data for the entire spectrum of the desired force velocities. The ramification of this finding is that a single experiment for the measurement of T_f cannot provide the required parameters for calibration. This further implies that the change in beat length is also dependent on the amplitude and velocity of loading. The suggested method takes both effects into account through empirical analysis of experimental data