Fiber loop ring-down cavity integrated U-bent single-mode-fiber for magnetic field sensing

A novel magnetic field sensing system based on the fiber loop ring-down technique is proposed in this paper. In the fiber loop, a U-bent single-mode-fiber structure coated with magnetic fluid(MF) serves as the sensing head, and an erbium-doped fiber amplifier(EDFA) is introduced to compensate for the intrinsic loss of the cavity. The ring-down time of the system varies with the change of applied magnetic field due to the tunable absorption coefficient and refractive index of the MF. Therefore, measurement of the magnetic field can be realized by monitoring the ringdown time. The experimental results show that the performance of the system is extremely dependent on the interrogation wavelength, because both the gain of the EDFA and the loss of the sensing head are wavelength dependent.We found that at the optimal wavelength, the ratio of the gain to loss attained its maximum. The sensing system was experimentally demonstrated and a sensitivity of-0.5951 μs∕Oe was achieved.

Finesse measurement for high-power optical enhancement cavity

Finesse is a critical parameter for describing the characteristics of an optical enhancement cavity(OEC). This paper first presents a review of finesse measurement techniques, including a comparative analysis of the advantages, disadvantages, and potential limitations of several main methods from both theoretical and practical perspectives. A variant of the existing method called the free spectral range(FSR) modulation method is proposed and compared with three other finesse measurement methods, i.e., the fast-switching cavity ring-down(CRD) method, the rapidly swept-frequency(SF) CRD method, and the ringing effect method. A high-power OEC platform with a high finesse of approximately 16000 is built and measured with the four methods. The performance of these methods is compared, and the results show that the FSR modulation method and the fast-switching CRD method are more suitable and accurate than the other two methods for high-finesse OEC measurements. The CRD method and the ringing effect method can be implemented in open loop using simple equipment and are easy to perform. Additionally, recommendations for selecting finesse measurement methods under different conditions are proposed, which benefit the development of OEC and its applications.

High-sensitivity methane monitoring based on quasi-fundamental mode matched continuous-wave cavity ring-down spectroscopy

Continuous-wave cavity ring-down spectroscopy(CW-CRDS)is an important technical means to monitor greenhouse gases in atmospheric environment.In this paper,a CW-CRDS system is built to meet the needs of atmospheric methane monitoring.The problem of mode matching is explained from the perspective of transverse mode and longitudinal mode,and the influence of laser injection efficiency on measurement precision is further analyzed.The results of cavity ring-down time measurement show that the measurement precision is higher when the laser is coupled with the fundamental mode.In the experiment,DFB laser is used to calibrate the system with standard methane concentration,and the measurement residual is less than±4×10^(-4)μs^(-1).The methane concentration in the air is monitored in real time for two days.The results show the consistency of the concentration changes over the two days,which further demonstrates the reliability of the system for the measurement of trace methane.By analyzing the influence of mode matching,it not only assists the adjustment of the optical path,but also further improves the sensitivity of the system measurement.

Measurement of OH Radicals in Dielectric Barrier Discharge Plasmas by Cavity Ring-Down Spectroscopy

Near-infrared continuous wave cavity ring-down spectroscopy was applied to mea- sure the OH radicals in dielectric barrier discharge plasmas, which play an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. The P-branches of OH X2YIi （vI ： 2 ＋-- it 0） bands were used for number density measurements. The OH number density and plasma temperature were determined for different applied voltages, gas pressures and concentrations of both oxygen and water. The temporal evolu- tion of the OH number density was obtained by using the ＂time window＂ method, which was used to extract individual ring-down times at different times in a half period of the sine wave applied voltage in dielectric barrier discharge plasmas.

Cavity Ring-Down Spectroscopy Measurements of Ambient NO3 and N2O5

NO3 and N2O5 are important participants in nocturnal atmospheric chemical processes,and their concentrations are of great significance in the study of the mechanism of nocturnal atmospheric chemical reactions.A two-channel diode laser based cavity ring-down spectroscopy(CRDS)instrument was developed to monitor the concentrations of NO3 and N2O5 in the atmosphere.The effective absorption length ratio and the total loss coefficient of the instrument were calibrated using laboratory standard samples.The effective absorption cross section of NO3 at 662 nm was derived.A detection sensitivity of 1.1 pptv NO3 in air was obtained at a time resolution of 1 s.N2O5 was converted to NO3 and detected online in the second CRDS channel.The instrument was used to monitor the concentrations of NO3 and N2O5 in the atmosphere of winter in Hefei in real time.By comparing the concentration changes of pollutants such as nitrogen oxides,ozone,PM2:5 in a rapid air cleaning process,the factors affecting the concentrations of NO3 and N2O5 in the atmosphere were discussed.

Cavity Ring-Down Spectroscopy Measurements of Trace H_(2)S in SF_(6) and SF_(6)/N_(2) Mixture

H_(2)S is one of the most important characteristic decomposition components of SF_(6)insulated gas,and the detection of trace H_(2)S is significant for early fault diagnosis of gas insulated electrical equipment.A 1578 nm wavelength distributed feedback diode laser(DFB-DL)based cavity ring-down spectroscopy(CRDS)experimental platform is developed to monitor the concentrations of H_(2)S in SF_(6)and SF_(6)/N_(2)mixture carrier gas.The detection sensitivity is higher than 1×10^(-6).The absorption cross section parameterσis vital for calculating the concentration.With repeated experiments using standard gas samples,parameterσof H_(2)S in pure SF_(6)and SF_(6)/N_(2)mixture carrier with different mixing ratios is calibrated.Compared with the simulatedσvalues,the influence of carrier gas on the broadening of spectral profile is discussed.The variation of absorption cross sectionσwith different carrier gas mixing ratios is studied as well,so that the calculation of the concentration in the carrier gas of any mixing ratio is possible.Thus,the application of CRDS in trace component detection of gas insulated electrical equipment is promising.

C2H2 Overtones Near 12300 cm^-1 Revisited with a Very Sensitive Cavity Ring-down Spectrometer

A cavity ring-down spectrometer （CRDS） is constructed with a single-mode continuous-wave Ti：Sapphire laser. It allows attaining a minimum detectable absorption of 1.8× 10^-10cm^-1. The spectrometer is applied to record the overtone spectrum of ^12C2H2 in the 12240- 12350 cm-1. Compared with the previous CRDS and intra-cavity laser absorption spectroscopy studies in the same region, the present measurement achieved better sensitivity and better precision as well. As a result, the ro-vibrational parameters of the high overtone bands of acetylene at 12290.12, 12311.82, and 12350.61 cm^-1 have been refined. The advantages of the present CRD spectrometer is also demonstrated by the newly observed and well characterized perturbation on the f component of the very weak band near 12289 cm^-1. The quantitative measurement capability of the spectrometer is verified with the measurement of the water lines and employed to give the absolute band intensities of those three acetylene bands.

Field Measurement of NO2 and RNO2 by Two-Channel Thermal Dissociation Cavity Ring Down Spectrometer

A two-channel thermal dissociation cavity ring down spectroscopy （CRDS） instrument has been built for in situ, real-time measurement of NO2 and total RNO2 （peroxy nitrates and alkyl nitrates） in ambient air, with a NO2 detection limit of 0.10 ppbv at 1 s. A 6-day long measurement was conducted at urban site of Hefei by using the CRDS instrument with a time resolution of 3 s. A commercial molybdenum converted chemiluminescence （Mo-CL） instrument was also used for comparison. The average RNO2 concentration in the 6 days was measured to be 1.94 ppbv. The Mo-CL instrument overestimated the NO2 concentration by a bias of ＋1.69 ppbv in average, for the reason that it cannot distinguish RNO2 from NO2. The relative bias could be over 100% during the afternoon hours when NO2 was low but RNO2 was high.

1.3 GHz连续波超导射频腔的数字低电平射频系统和腔模拟器设计及测试

1.3 GHz连续波超导射频腔需要高精度低电平射频(LLRF)系统来稳定超导腔的电磁场。但由于1.3 GHz CW射频腔的高负载品质因数和宽电磁频段,射频腔在频域中的电磁带宽较小。射频功率源与射频腔体之间的微小电磁频率差异易造成发生器驱动谐振器控制系统的不稳定,最终导致腔体电磁场的变化。开发了一种自激环控制系统,以防止“有质”不稳定性的发生,并补偿微噪声的影响。此外,还开发了数字1.3 GHz射频腔体模拟器,用于验证LLRF系统的设计算法。测试表明,即使在射频腔失谐5 Hz时,自激励控制系统也能确保腔场的稳定性。经过对比,验证了1.3 GHz射频腔体模拟器是测试新算法的可靠平台。

High Precision Cavity Ring Down Spectroscopy of 6υ3 Overtone Band of 14N2 16O near 775 nm

Transitions of the 6υ3 overtone band of ^14N2 ^16O near 775 nm have been studied by continuous-wave cavity ring-down spectroscopy. Line positions and intensities were derived from a fit of the line shape using a hard-collisional profile. The line positions determined with absolute accuracy of 5×10^-4 cm^-1 allowed us to reveal finer ro-vibrational couplings taking place after J〉14 except a strong anharmonic interaction identified by the effective Hamiltonian model. The absolute line intensities have also been retrieved with an estimated accuracy of 2% for a majority of the unblended lines. A new set of ro-vibrational and dipole moment parameters were derived from the experimental values. A comparison between the line positions and intensities of the 6υ3 band obtained in this work and those from previous studies is given.

Data point selection for weighted least square fitting of cavity decay time constant

For the accurate extraction of cavity decay time, a selection of data points is supplemented to the weighted least square method. We derive the expected precision, accuracy and computation cost of this improved method, and examine these performances by simulation. By comparing this method with the nonlinear least square fitting （NLSF） method and the linear regression of the sum （LRS） method in derivations and simulations, we find that this method can achieve the same or even better precision, comparable accuracy, and lower computation cost. We test this method by experimental decay signals. The results are in agreement with the ones obtained from the nonlinear least square fitting method.

976 nm类噪声锁模光纤激光器

输出波长为976 nm的脉冲激光器在科研及工业领域具有重要应用.利用非线性光纤环镜(nonlinear optical loop mirror, NOLM)搭建了“9”字型谐振腔,通过在环镜中加入长度为30 m的无源光纤,实现了类噪声脉冲(noise-like pulse, NLP)输出,丰富了976 nm波段的脉冲类型.脉冲的重复频率为4.79MHz,最大输出功率为13.35 mW,最大脉冲能量为2.79 nJ.进一步研究该NLP在不同泵浦功率的脉冲特性变化,结果表明,脉冲能量以及脉冲宽度均随着泵浦功率的增大而增大,但是其自相关曲线中的相干尖峰宽度随泵浦功率的增大而减小.该研究为976 nm类噪声脉冲的产生提供一种简单经济的方案,在超连续谱产生以及类噪声脉冲动力学研究方面具有潜在应用价值.

光纤腔衰荡光谱技术及其最新进展

阐述了腔衰荡光谱技术的基本原理及应用;从结构设计的角度考虑,归纳出光纤衰荡腔的三种基本结构,并对光纤腔衰荡光谱技术及其最新进展进行了评述;报道了我们采用脉冲光纤环衰荡光谱技术进行液体检测取得的最新进展;在总结光纤腔衰荡光谱技术优点的基础上,展望了其发展前景。

板材充液拉深动态加载系统开发与应用

液室压力和压边力加载路径是充液拉深工艺中决定零件成形质量的关键工艺参数。针对传统被动加压方式加载路径单一,液室压力较低,不能满足复杂结构零件和高强度材料的成形要求,提出采用闭环控制技术实时调节液室压力和压边力的动态加载方式。设计可控压边和高压增压系统,并应用基于上、下位机的检测控制系统实时调节凸模速度、压边力和液室压力满足设定的工艺参数。在动态加载系统上进行异型结构不锈钢零件的充液拉深试验,动态加载路径下试验件一次整体成形,且表面质量好,贴模精度高。结果表明,充液拉深动态加载系统能够实现复杂加载轨迹和局部高压,适用于复杂结构和高强度材料零件的成形制造。

8字形腔波长可调谐锁模脉冲光纤激光器

采用非线性光纤环形镜加脉冲锁模技术及可调谐光纤光栅滤波器,对8字形腔被动锁模掺铒光纤激光器进行了波长可调谐输出的实验研究.在EDFA抽运光功率一定的情况下,通过调节偏振控制器和可调谐光纤光栅滤波器,获得了光谱稳定、重复频率1.1MHz、输出功率起伏小于0.8dBm、中心波长在1548～1570nm内连续可调的短脉冲输出.该激光器可实现自启动锁模,且长时间稳定工作无需任何调整.

基于F-P干涉技术的通信波段可调谐激光器

根据F P干涉原理设计制作了楔型F P滤波器,使用这种滤波器进行可调谐掺铒光纤激光器实验,980nmLD泵浦功率为25mW时获得了稳定的单纵模激光输出,输出功率约0.6mW,线宽低于0.06nm,在34.2nm波长调谐范围内功率变化不超过0.5dB,边模抑制比大于35dB。

增大ECLD调谐输出曲线双稳环宽度的研究

利用导出的描述外腔半导体激光器 (ECLD)双稳特性的阈值载流子密度与振荡频率之间关系的基本方程 ,求得了跳变点处的载流子密度及频率的解析表达式 ,并由此分析了双稳环宽度随ECLD几个关键参量的大小而变化的情况。

高灵敏度光纤磁场传感器的设计与模型研究

根据磁致伸缩效应和光纤腔衰荡技术设计了高灵敏度的磁场传感器。基于Jiles-Atherton模型、二次畴转模型和光纤衰荡技术波长解调原理,建立了磁场传感器的数学模型,模型反映了磁场-应变-光纤腔衰荡时间之间的耦合机制。对不同预应力作用下Terfenol-D棒的磁致伸缩输出应变、衰荡时间、系统灵敏度与磁场之间的变化规律进行了理论分析和仿真研究,结果表明,该传感器具有很高的灵敏度,可通过改变预应力实现不同量程范围内的磁场测量,研究可为发展新型的高灵敏度磁场传感器提供了理论指导。

非线性放大环形镜“8”字腔光纤激光器实验研究

使用976nm半导体激光器作为抽运源,对利用非线性放大环形镜(NALM)锁模运行的掺Yb3+光纤激光器进行了实验研究,获得了脉冲宽度43.6ps,中心波长为1053.3nm的锁模脉冲激光输出,光谱宽度8nm,输出功率为0.2mW,重复频率为18.2MHz,经过放大,通过光栅对腔外色散补偿,在腔外产生了宽度616fs的脉冲激光·