CO2激光写入旋转折变型长周期光纤光栅的制作及理论分析

提出了一种光纤横截面折射率变化呈旋转非对称变化的长周期光纤光栅（R-LPFG） 结构,并利用多层圆波导理论和横截面折射率离散分析方法,结合模式耦合方程组和数值求解方法理论分析了这种光栅的模式耦合特征.理论分析表明R-LPFG纤芯基模主要与一阶非对称包层模发生耦合,当光栅旋转度逐渐变大时,R-LPFG基模会与一阶非对称包层模的奇模和偶模同时发生耦合,这就会使原来单一的谐振峰逐渐分裂成双峰,这是常规光栅类型所不具有的透射谱特征.由于R-LPFG的双峰来自同一对耦合模式,它们对温度的响应很相似,因此可利用双峰间 关键词： 光纤传感 光纤光栅 2激光')" href="#">CO₂激光 旋转折变     

Mode field profile and polarization dependence of long period fiber gratings written by CO2 laser

Mode field profiles and polarization dependence in the CO₂-laser-notched long period fiber gratings (LPFGs) were investigated in detail. Asymmetric field profile was demonstrated in the 2- and 3-dimensional far field patterns of the CO₂-laser-notched LPFG, which experimentally verifies the previously-presented asymmetry of refractive index modulation within the cross section of the gratings. Polarization dependent loss (PDL) of the LPFG was measured via Mueller matrix method. The maximum PDL of 1.68 dB and the maximum polarization extinction ratio of 1.07 dB are observed in the LPFG.     

Bend-insensitive long-period fiber grating sensors

A novel bend-insensitive long-period fiber grating (LPFG) sensor written using focused CO₂ laser pulses is demonstrated, for the first time, to our knowledge. It is found that the central wavelength shift of such a LPFG is only −0.018 nm even for a curvature of 1.1 m⁻¹ at the most bend-insensitive position of the LPFG. Experimental results show that the bend sensitivity of the central wavelength of the LPFG has a periodic distribution along its circular directions. Such a bend-insensitive sensor could be used to solve the problem of cross-sensitivity between bend and other measurands, such as temperature, strain or refractive index, which is an unsolved problem for LPFG sensors in practice. In addition, the bend sensitivity of the LPFG can be adjusted by selecting its circular positions.     

高频CO2激光脉冲写入的长周期光纤光栅传感器的特性研究

报道了一种用高频CO₂激光脉冲写入的新型长周期光纤光栅（LPFG）.发现该LPF G具有独特的应变、温度、弯曲、扭曲和横向负载特性.分析表明这些特性与单侧CO₂激光 加热导致光 栅横截面折射率分布不均匀有关.基于这些独特的特性提出了几种新型的LPFG传感器：1)能 彻底解决弯曲与其他参量之间交叉敏感问题的弯曲不敏感的传感器；2) 不仅能绝对测量扭 曲率而且能判断扭曲方向的扭曲传感器；3)单个LPFG实现温度和横向负载同时测量的传感器 .这些新型的传感器具有方法简 关键词： 长周期光纤光栅 光纤传感器 应变传感器 温度传感器 弯曲传感器 扭曲传感器     

Multi-Edge-Written Long-Period Fibre Gratings with Low PDL by Using High-Frequency CO2 Laser Pulses

A novel long-period fibre grating （LPFG） with low polarization-dependent loss （PDL） is fabricated by using a multi-edge exposure method with high frequency CO2 laser pulses. The experimental results show that the PDL of a triple-edge-written LPFG with a peak amplitude of-16.5 dB can be as low as 0.22 dB. These hovel LPFGs can find important applications in optical communication and sensing.     

Temperature and current coefficients of lasing wavelength in tunable diode laser spectroscopy

The factors determining temperature and current coefficients of lasing wavelength are investigated and discussed under monitoring CO₂-gas absorption spectra. The diffusion rate of Joule heating at the active layer to the surrounding region is observed by monitoring the change in the junction voltage, which is a function of temperature and the wavelength (frequency) deviation under sinusoidal current modulation. Based on the experimental results, the time interval of monitoring the wavelength after changing the ambient temperature or injected current (scanning rate) has to be constant at least to eliminate the monitoring error induced by the deviation of lasing wavelength, though the temperature and current coefficients of lasing wavelength differ with the rate.     

基于包层旋转折变型光纤光栅的动态增益均衡器

提出并利用高频CO₂激光脉冲在普通单模通信光纤上写出了包层旋转折变型长周期光纤光栅(R-LPFG). 在分析R-LPFG波导结构和双折射特性的基础上，对光栅周期数和折变旋转度分别为50和7.2°/周期的R-LPFG进行了实验研究.结果表明，这种类型的光栅可极大地降低其谐振波长对横向负荷不同作用方向所表现出的方向相关性，并且其谐振峰幅度横向负荷灵敏度在任意方向下均高达0.37dB/(g·mm^-1) ，是普通LPFG横向负荷灵敏度的9倍左右.利用R-LPFG的温度线性响应特性和独特的横向负荷特性，设计并实验研究了一种动态增益均衡器，用其平坦掺铒光纤放大器增益谱，在C波段32nm范围内，其平坦度小于±0.5dB，可满足实际环境对通信系统的平坦应用要求. 关键词： 光纤通信技术 长周期光纤光栅 动态增益均衡 旋转折变     

Structure design of refractive index sensor based on LPFG with double-layer coatings

Based on the coupled-mode theory, the mode transition and transmittance of LP modes in an LPFG with double-layer coatings are studied theoretically. As a refractive index sensor, this LPFG structure is designed for high sensitivity by selecting a suitable thickness and refractive index of the double-layer coatings. Date simulation indicates that the effective index of the LP cladding mode increases in steps with the first coating thickness, thus LP mode is guided to the overlay and the mode transition takes place, where the LP modes can be easily affected by the ambient. Thereby, the coated LPFG sensor should be designed to operate at the transition region. In addition, the effective refractive index of LP cladding modes also increases in steps with the refractive index of the second overlay. For the higher order LP modes, the transition regions are broaden and the dynamic responses are greater than the lower LP modes, which means the higher order LP modes can be used for lower index overlay sensing. Further, the transmittance spectrum, sensitivity and measurable dynamic range of this LPFG are analyzed. The sensitivity of the LPFG sensor is available to $10^{3}$ with suitable overlay thickness and measurable dynamic range of the sensitive film refractive index is available to $10^{-2}$ . So the presented LPFG sensor enhances the design flexibility and enlarges the scope of application, which will likely be developed for high sensitivity LPFG sensors.     

半导体的CO2激光退火和合金化

本文主要研究连续CO₂激光对半导体的照射效应。实验结果与理论分析说明,用连续CO₂激光照射可将半导体样片加热到所需的温度。与其它短波长的激光不同,波长为10.6μm的连续CO₂激光照射半导体有如下特点:CO₂激光是借助于自由载流子吸收与半导体耦合;样片在深度方向被均匀加热;激光背面照射可以增强退火效果。连续CO₂激光照射可以固相外延再生长的方式使As离子注入Si的损伤层退火恢复。在再生长的过程中注入的As离子进入替位,电激活率很高,而且不发生杂质再分布。将连续CO₂激光背面照射成功地应用于GaAsFET制备欧姆接触,既可避免激光正面照射对器件结构的破坏,又能得到比热退火为好的电学性能。 关键词：     

Highly sensitive fiber-optic torsion sensor based on an ultra-long-period fiber grating

A high sensitivity fiber-optic torsion sensor, which can measure twist rate and determine twist direction simultaneously based on a novel ultra-long-period fiber grating (ULPFG) with a period of up to several millimeters, is proposed and demonstrated. Such an ULPFG is fabricated by using the high-frequency CO₂ Laser pulses exposure technique. The unique torsion characteristics of the ULPFG are simply analyzed by using the mode coupling theory and the birefringence effect. The experimental results show that the high order resonant wavelengths of the ULPFG have higher torsion sensitivities, which is several times higher than that of the normal LPFG. In addition, an intensity-type demodulation approach used to realize real-time torsion measurement is proposed and demonstrated based on the edge filtering effect of the ULPFG.     

基于双芯光纤耦合器的梳状滤波器及其CO2激光调节

基于光纤耦合器的波长特性,分析了光纤耦合器作为梳状滤波器的光谱特性;将长度不同的双芯光纤(twin-core fiber,TCF)熔接在两根单模光纤(single mode fibors,SMF)之间,实验制得具有不同峰值波长间隔的基于双芯光纤耦合器的全光纤型梳状滤波器,其消光比可达25 dB. 首次使用CO₂激光对其进行光谱调节,调节导致光谱向长波方向漂移和消光比的变化. 这种调节是基于CO₂与光纤相互作用时的残余应力释放和熔融变形机理,使用氢载TCF可以在相同实验参数下得到更大的波长调节激光. 关键词： 双芯光纤耦合器 梳状滤波器 2激光')" href="#">CO₂激光 残余应力     

长周期光纤光栅弯曲特性的模式耦合理论分析

运用模式耦合理论分析了长周期光纤光栅的弯曲特性,得出了该特性与光纤归一化频率V和模式的阶数m有关的结论,即随着弯曲曲率的增大,其谐振波长可能向长波方向漂移也可能向短波方向漂移,光纤的归一化频率和所考察模式的阶数是其决定因素.并运用等效倾斜光栅理论合理解释了弯曲长周期光纤光栅耦合峰幅值的变化特性.     

DFB激光二极管电流-温度调谐特性的解析模型

从DFB型激光二极管调谐机理出发,提出了电流-温度调谐特性的解析模型,通过实验测量结果辨识出模型参数,将模型应用于四个不同厂家的DFB型二极管激光器,得到激光器电流-温度调谐的解析模型;将模型预测值与实验测量值比较,相关系数均在0.9999以上.同时,利用CO₂气体的多个吸收谱线测量激光的波长,验证了解析模型的预测波长值,与HITRAN谱库中CO₂气体吸收波长的误差在3 pm内.解析模型能够精确预测激光器在快速调谐过程中的瞬态输出波长,其精度能够满足光谱分析、光 关键词： DFB激光二极管 调谐机理 解析模型 电流调谐和温度调谐     

Theoretical prediction on the wavelength-temperature shift in pulsed TE CO2 lasers

The wavelength-temperature shift observed in pulsed TE CO₂ lasers is discussed theoretically by means of Six-temperature model rate equations for tunable TE CO₂ lasers. Numerical calculations of the temperature-wavelength shift in a pulsed TE CO₂ laser with a simple plano-concave stable resonator, whether excited by conventional low-inductance fast-discharge scheme or by a long-pulse Pulser/sustainer discharge scheme, show that the laser output wavelengths are within the 10P branch as the ambient temperature varies from 228 to 338 K, but will change as the ambient temperature varies. The laser output wavelengths will move to the transition lines with longer wavelengths in the 10P branch as the ambient temperature increases and vice versa. The calculated results also illustrate that near the ambient temperature of 310 K, the laser is more likely to operate on multi-transition lines. Considering this wavelength-temperature shift, the chilling device adopted in high-power high repetition rate TE CO₂ lasers is important in maintaining a stable laser output spectra as well as a stable laser output power. The numerical results also suggest that a frequency agile resonator is highly recommended if stable laser output spectra are required in TE CO₂ lasers.     

Step-changed period chirped long-period fiber gratings fabricated by CO2 laser

A CO₂ laser and point-by-point method are used for fabricating step-changed period chirped long-period fiber gratings (LPFG). Several types of period chirped LPFGs have been demonstrated, such as, linearly chirp, peak-shape chirp, and cascaded linearly chirp. Unlike uniform LPFGs, the spectrum change such as multiple attenuation peaks, broader spectrum can be seen in these chirp gratings, and the spectral shape can be controlled by the grating period. Especially, the cascaded linearly chirped LPFGs performs a multi-peak as interference between the core mode and cladding mode, which can be used as multi-wavelength filters in fiber optic communication and fiber optic sensors. Also, a linear tuning range of 1.6 nm with -0.559 pm/με tuning rate is achieved in these types of devices by applying an axial strain.     

Sensitive detection of temperature behind reflected shock waves using wavelength modulation spectroscopy of CO2 near 2.7?μm

Tunable diode-laser absorption of CO₂ near 2.7 μm incorporating wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) is used to provide a new sensor for sensitive and accurate measurement of the temperature behind reflected shock waves in a shock-tube. The temperature is inferred from the ratio of 2f signals for two selected absorption transitions, at 3633.08 and 3645.56 cm⁻¹, belonging to the ν ₁+ν ₃ combination vibrational band of CO₂ near 2.7 μm. The modulation depths of 0.078 and 0.063 cm⁻¹ are optimized for the target conditions of the shock-heated gases (P∼1–2 atm, T∼800–1600 K). The sensor is designed to achieve a high sensitivity to the temperature and a low sensitivity to cold boundary-layer effects and any changes in gas pressure or composition. The fixed-wavelength WMS-2f sensor is tested for temperature and CO₂ concentration measurements in a heated static cell (600–1200 K) and in non-reactive shock-tube experiments (900–1700 K) using CO₂–Ar mixtures. The relatively large CO₂ absorption strength near 2.7 μm and the use of a WMS-2f strategy minimizes noise and enables measurements with lower concentration, higher accuracy, better sensitivity and improved signal-to-noise ratio (SNR) relative to earlier work, using transitions in the 1.5 and 2.0 μm CO₂ combination bands. The standard deviation of the measured temperature histories behind reflected shock waves is less than 0.5%. The temperature sensor is also demonstrated in reactive shock-tube experiments of n-heptane oxidation. Seeding of relatively inert CO₂ in the initial fuel-oxidizer mixture is utilized to enable measurements of the pre-ignition temperature profiles. To our knowledge, this work represents the first application of wavelength modulation spectroscopy to this new class of diode lasers near 2.7 μm.     

Fluorescence spectroscopy of anisole at elevated temperatures and pressures

Laser-induced fluorescence of anisole as tracer of isooctane at an excitation wavelength of 266 nm was investigated for conditions relevant to rapid compression machine studies and for more general application of internal combustion engines regarding temperature, pressure, and ambient gas composition. An optically accessible high pressure and high temperature chamber was operated by using different ambient gases (Ar, N₂, CO₂, air, and gas mixtures). Fluorescence experiments were investigated at a large range of pressure and temperature (0.2–4 MPa and 473–823 K). Anisole fluorescence quantum yield decreases strongly with temperature for every considered ambient gas, due to efficient radiative mechanisms of intersystem crossing. Concerning the pressure effect, the fluorescence signal decreases with increasing pressure, because increasing the collisional rate leads to more important non-radiative collisional relaxation. The quenching effect is strongly efficient in oxygen, with a fluorescence evolution described by Stern–Volmer relation. The dependence of anisole fluorescence versus thermodynamic parameters suggests the use of this tracer for temperature imaging in specific conditions detailed in this paper. The calibration procedure for temperature measurements is established for the single-excitation wavelength and two-color detection technique.     

Current analysis of DC negative corona discharge in a wire-cylinder configuration at high ambient temperatures

To study the characteristics of DC negative corona discharge in a wire-cylinder configuration at an ambient temperature range of 350–850 °C, the I–V characteristics and the current composition are analyzed under different conditions. A simple method is proposed to determine the DC corona onset threshold voltage. At high ambient temperatures, in the DC negative corona discharge gap, some electrons are not attached to the electronegative gas molecules and move to the anode tube. Thus, these electrons form an electron current, which may account for most of the total discharging current. The ratio of the electron current to the total discharging current increases with increasing temperature. In a mixture of O₂ and N₂ and a mixture of CO₂ and N₂, the ratio of electron current increases with increasing N₂ content in the mixtures. The cathode material has little influence on the corona discharge characteristics at high ambient temperatures.     

基于模式转换及双峰谐振效应的薄包层镀膜长周期光纤光栅特性研究

基于耦合模理论,首先研究了镀膜长周期光纤光栅(LPFG)高阶包层模的模式转换,划分了高阶包层模的非模式转换区及模式转换区。分析了高阶包层模有效折射率随薄膜厚度增加的响应特性,包层模谐振波长在模式转换区的偏移量要大于非模式转换区。在此基础上,研究了不同包层半径下高阶包层模谐振波长随光栅周期的变化情况,结果表明,相同包层半径下模式转换区内双峰间距的偏移量大于非模式转换区;无论在模式转换区还是非模式转换区,包层半径的减小将增加双峰间距的偏移量。最后分析了不同包层半径下的高阶包层模双峰透射谱在模式转换区及非模式转换区内的折射率响应,进而提出了薄包层镀膜LPFG的优化设计方案,当选定敏感膜层厚度及折射率处于镀膜LPFG的模式转换区内,光栅周期靠近相位匹配转折点时,将得到灵敏度高于传统LPFG双峰传感器的镀膜LPFG折射率型双峰传感器;而减小包层半径,将进一步提高传感器的分辨本领。     

Theoretical Research on Sensitivity for Resonance Wavelength of Long Period Fiber Grating

In a long period fiber grating(LPFG), the forward-propagating guided mode in the core is coupled to a number of co-propagating lossy cladding modes at resonance wavelengths. Sensitivity of resonance wavelength against temperature, axial strain, and external refractive index is modeled in this paper. Schemes for temperature compensation and enhanced wavelength tuning of LPFG are proposed and analyzed. Experiments show accordant results with simulations. Temperature coefficient of the LPFG compensated by means of axial strain is below 1.8 nm/100℃. For the LPFG with polymer recoating layer, the enhanced tuning range is more than 70 nm.