Interrogation technique for a fiber Bragg grating sensing array based on a Sagnac interferometer and an acousto-optic modulator

We propose and experimentally demonstrate a novel real-time interrogation technique for a fiber Bragg grating (FBG) sensing system that is based on a frequency-shifted asymmetric Sagnac interferometer. FBG sensors are connected to the Sagnac loop by an optical coupler, and an acousto-optic modulator (AOM) is asymmetrically placed in the Sagnac loop. By linearly sweeping the driving frequency of the AOM, the environmental variation around each FBG sensor can be determined by measuring the spectrum of the interference signals of the two counterpropagating light beams reflected by the corresponding FBG. The system has the advantages of low cost and real-time sensing.     

Signal-to-noise ratio of ultrashort pulses enhanced by a fiber Sagnac loop

A method to enhance the signal-to-noise ratio of ultrashort pulses is presented with picosecond pulses as an example. Numerical simulation and analysis show that the signal-to-noise ratio can be improved by four orders of magnitude using a fiber Sagnac loop with two different fibers. It is also found that there is an optimal length of the fiber for improving the signal-to-noise ratio. This is a useful method for improving the signal-to-noise ratio of the seed light in the chirped pulse amplification (CPA) or optical parametric chirped pulse amplification (OPCPA) systems.     

基于长周期光纤光栅和保偏光纤Sagnac环光谱特性的温度与应变测量研究

提出并研制了一种结构简单、成本低廉的温度与应变同时测量系统,其结构是在保偏光纤Sagnac环内接入一个长周期光纤光栅(LPFG)。利用LPFG对保偏光纤Sagnac环的透射光谱进行调制,通过监测谐振峰波长和强度的变化,发现波长随温度和保偏光纤的应变变化,强度随LPFG的应变变化,因此可以实现温度与应变的区分测量,并且可判断出应变的施加位置。实验得到该系统的温度灵敏度为0.181 81 nm·℃-1,LPFG区的应变灵敏度为0.005 283 dB·με-1,保偏光纤Sagnac环区的应变灵敏度为0.015 72 nm·με-1。实验结果表明该方案可行,并具有一定的实用性。     

基于PDMS材料的Sagnac干涉光纤挥发性有机物传感器

在一段8cm长的保偏光纤两端分别熔接两段普通的单模光纤,在保偏光纤的侧面均匀地镀上一层聚二甲基硅氧烷材料,聚二甲基硅氧烷材料经该段保偏光纤接入到一个光纤耦合器中,从而形成一个光纤Sagnac干涉仪.聚二甲基硅氧烷材料吸附挥发性有机物分子时,会引起聚二甲基硅氧烷材料体积上的膨胀,导致Sagnac干涉波长的漂移,通过对Sagnac干涉光波长漂移的测量即可实现对挥发性有机物气体的检测.实验测量了传感器对挥发性有机物浓度的响应,结果表明,在0~6 000ppm浓度范围内,传感器的灵敏度为1.03pm/ppm,由光谱仪的最小分辨率为0.02nm可知,该传感器对挥发性有机物的检测下限约为19.4ppm.该传感器相比聚二甲基硅氧烷材料与光纤光栅结合的传感器,灵敏度提高了4 300倍.     

Increase in the correlation length of nonmonochromatic radiation during propagation in a single-mode optical fiber containing random inhomogeneities and the influence of this increase on the operation of a fiber ring interferometer

An analysis is made of the conditions under which the coupling of orthogonal polarization modes at random inhomogeneities in single-mode optical fibers leads to an increase in the correlation length of a source of nonmonochromatic radiation. It is shown that when long-base fiber ring interferometers with a single-mode fiber ring system possessing weak linear birefringence are used, the correlation length of the nonmonochromatic radiation at the interferometer exit is increased, which means that the interference pattern can have satisfactory visibility even when there is an appreciable difference between the interferometer arms as a result of the Sagnac effect due to the Earth’s rotation. The calculations were made by mathematical modeling of random inhomogeneities in the fiber. Zh. Tekh. Fiz. 69, 140–143 (July 1999)     

2.1 μm可调谐多波长掺钬光纤激光器

为了实现2.1 μm波段光纤激光器输出多波长激光，设计了一种基于光纤Sagnac干涉仪的可调谐多波长掺钬光纤激光器。采用1.9 μm波段掺铥光纤激光器泵浦一段长3 m的掺钬石英光纤，获得2.1 μm波段的光放大；环形腔中，由保偏光纤和偏振控制器构成的光纤Sagnac干涉仪，实现2.1 μm波段周期滤波，获得了2.1 μm波段多波长激光，输出功率1 mW~15 mW可调谐，最多可观测到6个波长的激光输出。通过调节环形腔内偏振控制器，能够实现2.1 μm波段1~6个波长的调谐。     

A distributed frequency modulation continuous wave fiber stress sensor based on a birefringent sagnac ring configuration

A new distributed stress sensor, based on a birefringent fiber Sagnac ring configuration and frequency modulation continuous wave technology, has been investigated. The two forward-coupled mode beams in the Sagnac ring, induced by an applied stress at any given point along the fiber length, produce a beat frequency which can be analysed to determine, simultaneously, the magnitude and position of the stress. The system has a reasonable spatial resolution of 1 m in a dynamic sensing range of 50 m.     

Fiber optic sensor based on a sagnac interferometer including a recirculating-ring delay line with an edfa

We present a theoretical and experimental investigation of a Sagnac interferometer incorporating a fiber optic recirculating-ring delay line with an erbium-doped fiber amplifier to increase the effective length of the Sagnac loop and thereby improve the low-frequency response. Theoretical calculations show that the low-frequency response of the interferometer is enhanced as expected. However, the noise penalty of using a fiber amplifier in the ring is quite high, especially at low frequencies. The signal-to-noise ratio at low frequencies, using a superfluorescent erbium fiber source, is demonstrated as increasing by a factor of 2 compared to a single-loop Sagnac interferometer with the same total length of fiber, but without fiber amplifier.     

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.     

单轴分布式光纤传感器管线泄漏探测方法及定位理论分析

利用Sagnac干涉原理的分布式光纤传感器是一种可用于长途油气管线侵入、泄漏探测和定位的新技术.为了克服屏蔽和隔离Sagnac光纤环中未用作传感的那一半光纤所带来的屏蔽困难、成本增加等问题,提出一种单轴分布式光纤传感器的方法.其原理是利用一条光纤代替光纤环,并在光纤尾端设置法拉第旋转镜,构成偏振无关Sagnac干涉仪探测和定位管线泄漏.文中详细分析和讨论了该方法的系统光路和探测定位原理.     

Multiwavelength Linear-Cavity Fiber Laser Source Using a Sagnac Interferemetric Filter and a Strain-Induced Chirped Fiber Grating

A novel multiwavelength linear-cavity fiber laser source using a strain-induced chirped fiber Bragg grating, a Sagnac interferemetric filter, and Erbium-Ytterbium codoped fibers is proposed and demonstrated for the first time. The Sagnac interferemetric filter, which acts as a full-reflecting mirror, consists of a polarization-maintaining directional coupler and two pieces of polarization-maintaining fibers. The strain-induced chirped FBG, which acts as a partial-reflecting mirror, is achieved by bending a uniform FBG bonded at a slant onto the lateral surface of a simply supported beam. By tuning the strain-induced chirped FBG, desirable lasing wavelength and number can be easily achieved.     

Double random mirror Hi–Bi photonic crystal fiber Sagnac based multiwavelength fiber laser

In the present work, a multiwavelength Raman fiber laser based in a Sagnac configuration is presented. The Sagnac configuration is composed of a Hi–Bi photonic crystal fiber in the middle of two dispersion compensation fibers. The pump power induces two random mirrors to work in opposite directions in the Sagnac, leading to the amplified signal and the multiple scattering signals from the random mirror traveling through the Hi–Bi photonic crystal fiber in a counter-propagating way interfering in the output port.     

Fiber Sagnac Loop Filter with a Symmetrical Piecewise Uniform FBG

We propose a novel comb filter by using a symmetrical piecewise uniform fiber Bragg grating (S-PUFBG) in a Sagnac interferometer. The filter has wide bandwidth, equal passband spacing, and low insertion loss. The total transmission bandwidth is determined by the number and the parameters of the grating segments, while the transmission spectrum can be cosine modulated to realize comb-like filtering, and the modulating frequency can be controlled by adjusting the fiber length difference in the Sagnac loop.     

Photonics True-Time-Delay Unit for Phased-Array Antenna Using Multiwavelength Fiber Laser Based on a Sagnac Interferemetric Filter

A photonics true-time-delay system for phased array beamforming using a chirped grating-based time-delay element and a novel multiwavelength erbium-doped fiber ring laser source employing a Sagnac filtering mirror is proposed. The Sagnac filtering mirror consists of a polarization maintaining directional coupler and two pieces of polarization maintaining fibers. The lasing wavelength and number are determined by properly adjusting the polarization controller within the unidirectional ring cavity and are very stable. A chirped-grating-based true-time-delay system using the proposed laser source is constructed and demonstrated experimentally.     

Measurement of the chromatic dispersion of an optical fiber by use of a Sagnac interferometer employing asymmetric modulation

We describe a novel method, based on asymmetric modulation in a Sagnac interferometer, that measures the chromatic dispersion of single-mode fibers. The interferometer incorporates a phase modulator and a test fiber, so the dispersion can be determined from the interference fringe seen when a sweep rf signal is applied to the modulator. This technique provides picosecond temporal resolution without the need for any fast diagnostic equipment and is capable of accurately measuring the average dispersion of fibers several kilometers long.     

Comparisons of multi-wavelength oscillations using Sagnac loop mirror and Mach-Zehnder interferometer for ytterbium doped fiber lasers

A multiwavelength Ytterbium-doped fiber ring laser operating at 1030 nm region is demonstrated using a Sagnac loop mirror and a Mach-Zehnder interferometer. We report the Performance comparisons of multi-wavelength oscillations in Yb³⁺ doped fiber lasers (YDFL) with typical commercial ytterbium doped silica fibers. By adjusting the polarization controller (PC), a widely tunable laser range of 22 nm from 1030 nm to 1050 nm is obtained. The Mach-Zehnder interferometer (MZI) design has exhibited simplicity in the operation for controlling the smallest wavelength spacing compared to Sagnac loop mirror method. In our observations, the smallest achieved stable wavelength spacing in Sagnac loop mirror setup and MZI setup were 2.1 nm and 0.7 nm, respectively. In case of nine-wavelength operation with a MZI setup, the stability, Full Width at Half Maximum (FWHM) and side mode suppression ratio (SMSR) of laser lines are not affected by increasing pump power, While for above four wavelength operation, the laser stability with Sagnac loop mirror becomes worse specially for higher input pump power and the power fluctuation among the wave-lengths would be also slightly larger.     

Stable multiwavelength fiber ring laser with equalized power spectrum based on a semiconductor optical amplifier

We experimentally demonstrated a new structure of a multiwavelength semiconductor optical amplifier (SOA) ring laser based on a fiber Sagnac loop filter that can generate up to 25 stable output lasing wavelengths at room temperature. By varying the length of a polarization-maintaining (PM) fiber within the Sagnac loop filter, the wavelength spacing between the output lasing wavelengths can be changed to a desired value. By tuning a polarization controller (PC) within the Sagnac loop filter, stable multiwavelength 1550-nm operation with up to 17 lasing lines within 3 dB power level variation and with a wavelength spacing of ∼0.8 nm was achieved. The optical signal-to-noise ratios (OSNRs) of all the lasing wavelengths are greater than 40 dB.     

Pressure and temperature characterization of two interferometric configurations based on suspended-core fibers

In this work, two all-fiber interferometric configurations based on suspended core fibers (SCF) are investigated. A Fabry–Pérot cavity (FPC) made of SCF spliced in-between segments of single-mode and hollow-core fiber is proposed. The interferometric signals are generated by the refractive-index mismatches between the two fibers in the splice region and at the end of the suspended-core fiber. An alternative sensing head configuration formed by the insertion of a length of SCF as a birefringence element in a Sagnac loop interferometer is also demonstrated. In this structure, the interferometric signals are generated by interfering two counter propagating beams with different polarization states which propagate through a length of SCF as a birefringence element. The sensitivity to pressure and temperature was determined for both configurations. The results show that the pressure sensitivities are ? 4.68 × 10^? 5 nm/psi and 0.032 nm/psi for FPC and Sagnac loop interferometers, respectively. The temperature sensitivity of both structures has been obtained and the results have been discussed.     

主动相位偏置折叠型萨尼亚克光纤传感阵列时分复用技术

针对折叠结构萨尼亚克(Sagnac)光纤传感阵列存在噪声光与信号光混叠的问题,提出了一种主动相位偏置时分复用方案.在传统梯形结构传感阵列的基础上,通过调整总线光纤的长度关系和附加延迟光纤的方法,使噪声光和信号光依次交替返回而不会发生混叠.进一步分析表明,通过调整延迟光纤环的长度,可以使输入光脉冲的重复频率达到标准时分复用系统重复频率的二分之一.整个阵列的相位偏置由一个与输入光脉冲同步的相位调制信号驱动集成光学芯片实现.实验演示了一个两基元的传感阵列,最小时间间隔为331.25 ns,输入光脉冲重复频率可达754.727 kHz,在5 kHz处相位灵敏度为7.3μrad√(Hz),探头间串扰约为-51.75 dB.     

Sagnac quantum key distribution over telecom fiber networks

We present a new concept for compensation of single mode fiber (SMF) birefringence effects in a Sagnac quantum key distribution (QKD) setup, based on a polarization control system and a polarization insensitive phase modulator. Our experimental data show stable (in regards to birefringence drift) QKD over 1550 nm SMF telecom networks in Sagnac configuration, using the BB84-protocol [C.H. Bennett, G. Brassard, in: Proceedings of the IEEE International Conference on Computers, Systems, and Signal Processing, Institute of Electrical and Electronic Engineers, New York, 1984, p. 175] with phase encoding. The achieved total Sagnac transmission loop distances were between 100 km and 150 km with quantum bit error rates (QBER) between 5.84% and 9.79% for the mean-photon-number μ = 0.1. The distances were much longer and rates much higher than in any other published Sagnac QKD experiments. We also show an example of our one-decoy state protocol implementations (for the 45 km distance between Alice and Bob, corresponding to the 130 km total Sagnac loop length), providing an unconditional QKD security. The measurement results have showed feasibility of QKD over telecom fiber networks in Sagnac configuration, using standard fiber telecom components.