异步采样全保偏光纤光梳系统研制

为了获得长期稳定的光梳光源,利用压电陶瓷（PZT）和步进电机双级反馈控制方案,研制了两台基于非线性放大环形镜（NALM）锁模的异步采样光纤光梳系统。研究表明：该系统重复频率为75 MHz,5 min内重复频率的锁定峰峰值为±2 mHz,标准差为0.70 mHz,90 h内锁定峰峰值为±10 mHz,标准差为1.26 mHz;光纤光梳输出端口的脉冲平均功率为30 mW,经3 m保偏单模光纤压缩后脉冲宽度约为90 fs;激光器的重复频率差Δf在1 Hz～500 kHz范围内连续可调,当异步采样频差Δf为80 Hz时,扫描周期为12.5 ms,可探测出信噪比为6.3的太赫兹信号。该方案避免使用传统机械延迟线,具有重复频率差精确可调、采样速度快、抗干扰能力强等优点。     

NALM锁模全保偏光纤掺铒光学频率梳

飞秒光学频率梳是当今激光技术领域的重要研究方向。实验基于非线性放大环形镜(NALM)锁模激光器实现了全保偏光纤结构的掺铒光学频率梳。在基于NALM锁模的光纤激光器内部加入非互惠相移器,降低了锁模阈值,实现了超短脉冲激光器的自启动。经过脉冲放大和压缩,脉冲的峰值功率可达61.3 kW。将此高功率超短脉冲注入55 cm的保偏高非线性光纤(PM-HNLF)中,激光器的输出光谱被拓展至一个倍频层(1 030～2 200 nm)。辅以f-2f自参考探测技术,成功探测到了信噪比高达40 dB、线宽为40 kHz的载波包络偏频信号(f_0)。此外,通过使用两套电路反馈系统,将f_0信号与激光器重复频率信号(f_r)的频率抖动量分别降低至521.71 mHz和240μHz,实现了相位稳定的掺铒光学频率梳。     

用于激光频率参数测量的飞秒光学频率梳

近年来随着光纤制造技术和飞秒激光技术的成熟,以掺铒(Er)光纤光学频率梳为代表的频率梳技术,逐步突破了光学 频率测量领域,在长度测量、精密光谱分析、超低相位噪声微波频率产生、精密时间频率传递、温度测量等领域发挥出越来越重 要的作用,已成为许多高端科研领域的基础性工具。 但飞秒光学频率梳所解决的重要问题是对激光频率进行测量。 本文主要 面向激光频率参数测量的需求,研制基于掺 Er 光纤飞秒激光器的光学频率梳,在实现光学频率梳稳定运转的前提下,通过非线 性光学频率变换技术,实现光谱范围从掺 Er 光纤光学频率梳的中心波长向各个待测激光波长的转换,并完成与多个不同波长 激光的拍频信号探测。 目前已验证的飞秒光梳可测频率范围为 500 ~ 2 000 nm;频率稳定度和准确度为 10 -16 量级;线宽为 Hz 量级。 该指标满足了激光频率特性参数测量的需求,为激光绝对频率、频率漂移、线宽等参数的测量提供了基础性的测量工具。     

太赫兹双梳光谱中重复频率差影响的研究

太赫兹双梳光谱技术因其高频率分辨率和高灵敏度等优点,近年来成为一种有力的光谱测量技术。为了提高光谱系统的探测性能,本文分析了双梳光谱技术在时域和频域中的采样原理及方法,基于两台飞秒激光器搭建了一套重复频率可调的太赫兹双梳光谱系统。通过改变一台光频梳的重复频率,系统地研究了不同重复频率差对太赫兹双梳光谱系统性能的影响。结果表明有效范围内的重复频率差越小,探测到光谱质量越高。当重复频率差为10 Hz时,太赫兹双梳光谱系统的探测性能最佳。此研究为太赫兹双梳光谱技术选择最合适的重复频率差提供了方法。     

被动型CPT原子钟光源系统的设计与实现

简要介绍了相干布局数囚禁(CPT)原子钟的机理,分析了典型的CPT原子钟的系统构成.在此基础上,针对CPT原子钟的特点,以Rb原子钟为设计对象,设计并制作了一种可出射稳定激光频率并能与其他CPT原子钟相兼容的光源系统.该光源系统采用热电致冷器(TEC)对激光器进行温度控制,利用直接数字频率合成(DDS)技术和锁定放大(LIA)技术实现激光器光频稳定.测试结果表明,光源系统温度对波长的控制精度为0.001 2 nm,在此条件下,系统可以很好地完成激光光频的锁定.     

飞秒激光频率梳精密测距技术综述

高精度绝对距离测量在科技发展中具有重要的作用,传统的测量方法已无法满足日益提高的测量要求。光学频率梳是重频和相位完全锁定的锁模脉冲激光,具有很高的空间、时间和频率分辨率。基于光频梳的测距技术由于在大尺寸计量及未来空间任务中的应用潜力引起了极大关注。介绍了光频梳测距技术的发展与现状,重点介绍了5种测量原理,包括合成波长干涉法、多波长干涉法、色散干涉法、双光梳干涉法和飞行时间法。     

飞秒激光光学频率梳装置

“飞秒激光光学频率梳装置”（简称“飞秒光梳装置”）可用于对时间、频率和长度的超高精度测量，在导航定位、引力波探测、光通信等领域有着重要的作用。由中国计量科学研究院承担的科技部科技基础性工作专项基金项目“飞秒激光光学频率梳的研究”，9月12日通过了由国家质检总局组织的成果鉴定。     

定常及非定常射流泵性能及管道摩阻试验研究

探讨非恒定条件下射流泵性能及与频率有关的管理摩阻损失,并与恒定流进行对比试验。试验结果表明,在相同的条件下,非恒定流射流效率比恒定流效率高。在频率f=1-2Hz范围内,频率越高,管道摩阻损失越大。     

干涉条纹相位锁定系统

干涉曝光系统中干涉条纹的相位漂移会导致曝光对比度降低,为了有效抑制相位漂移,利用声光调制器对干涉光频率进行实时调制。分析了条纹漂移的特点,指出了主要干扰源是0~5 Hz的空气扰动。应用数值分析法得到了条纹漂移量与曝光对比度的关系曲线,并以此为依据提出了条纹锁定精度的目标值。针对所要达到的锁定精度,给出了系统硬件的选型方法,搭建了基于RTX的干涉条纹相位锁定系统。利用闭环辨识的方法得到了系统的参数模型,完成了反馈控制器的设计,最终实现了实时锁定条纹相位的功能。实验结果表明,在400Hz的控制频率下,干涉锁定系统能够有效抑制0~5Hz的低频扰动,干涉条纹相位漂移的3σ值可以控制在±0.04个条纹周期内,满足干涉光刻的曝光对比度要求。     

基于FPGA的谐振悬臂梁传感器智能接口电路

设计了基于FPGA的谐振式微悬臂梁传感器接口电路,将接口电路与谐振式微悬臂梁传感器组成闭环自激振荡系统.为满足接口电路智能化与输出频率稳定性的要求,设计了带有自动扫频功能的锁相环,以此锁相环为核心并结合外围放大、滤波、移相、整形电路构成的接口电路系统,能够自动扫频寻找谐振点并锁定,扫频精度为0.012 Hz左右,谐振频率的波动范围可达0.01Hz左右,且锁定范围极广.分别测试了谐振点在100kHz与500 kHz附近的两组传感器,该系统均能很好地自动扫频寻找谐振点并锁定.     

Experiment on the characteristics of torsional vibration of rotor-to-stator rub in turbomachinery

By use of the DK-II torsional vibration measuring system, the characteristics of torsional vibration of a rotor with rotor-to-stator rub are studied. It can be concluded that the measured natural torsional frequency (f_rub) with rub is higher than the one (f_ori) without rub. When rub happens, if the rotating frequency is higher than f_rub, torsional vibrations with a measured natural torsional frequency (f_rub) and 1×, 2× are excited; if the rotating frequency is between f_rub and f_ori, the torsional vibration which is so called resonance (one-time rotating frequency) is excited; if the rotating frequency is lower than f_ori, torsional vibrations with one-time rotating frequency and f_rub are excited, and a small torsional vibration with 2× is excited too.     

Eddy-Current Studies of Aging Processes in N26T3 Metastable Austenite Steel

In the N26T3 austenite steel, the eddy-current parameter f ₀ has been measured after one aging cycle at temperatures of 700 and 650°C, and after two aging cycles, one of them at the same temperature and the second at 600 and 550°C. The two-stage aging is conducted by two schemes: (1) preliminary aging at 700 or 650°C, transfer of samples without cooling to the room temperature into a furnace heated to 600 or 550°C, then isothermal aging and cooling to room temperature T _r; (2) the same operations as in the first scheme, but the samples are cooled to T _r after the preliminary aging. The eddy-current parameter f ₀ measured at T _r increases with time after aging at 700 and 600°C owing to isothermal martensite transformation. After the two-stage aging, the isothermal martensite transformation at T _r still takes place, but it is stabilized, i.e., the parameter f ₀ drops with time. The stabilization of the austenite is the more pronounced, the lower the temperature of the second stage of aging, and it is stronger after the two-stage aging by the second scheme.     

Effect of structure of aging metastable austenitic steels on the signal from an eddy-current transducer

Eddy-current parameterf ₀ of the N26T3 steel has been studied as a function of both the aging temperatureT _ag=20–800°C and the time τ of exposure to a constant temperature of 550 and 600°C up to 6h. In the initial state, the steel had two phases: (1) cooling-induced martensite+austenite (α+γ) or (2) strain-induced martensite+austenite (α′+γ). The parameterf ₀ drops monotonically as τ increases, and this drop is the faster, the higherT _ag. The parameterf ₀ changes nonmonotonically with the aging temperature. In addition to the initial two-phase structures, the one-phase γ structure has also been studied. The parameterf ₀ grows monotonically with the plastic cold strain and changes nonmonotonically with the aging temperature (20–800°C). Observed changes inf ₀ have been explained.     

Vibration response analysis caused by rubbing between rotating blade and casing

Two types of blade-tip rubbing due to the static misalignment of the bladed-disk center and casing center and casing deformation are simulated. By applying aerodynamic load in the blade lateral/flexural direction, vibration responses due to blade-casing rubbing are analyzed under the run-up process with constant angular acceleration and the steady-state process at 10000 rev/min. The effects of some parameters, such as the static misalignment e _c, casing stiffness k _c and casing deformation n _p, on the system vibration responses are also illustrated by spectrum cascades, time-domain waveforms of displacement, normal rubbing forces, amplitude spectra and the impulse P in a single blade-casing rubbing period. The results show that blade-tip rubbing will cause amplitude amplification and harmonic resonance phenomena when the multiple frequencies (nf _r) of rotational frequency (f _r) coincide with the first three flexural dynamic frequencies of the blade (f _n1, f _n2 and f _n3). For example, the displacement amplitudes at 3f _r, 14f _r and 38f _r are large and the vibration is dominant near f _n1. In addition, the casing deformation mainly excites the dominant Blade passing frequency (BPF), which is related to the casing deformation coefficient n _p. By comparing these impulse values, for the selected parameters in this paper, the casing stiffness has a greater effect on impulse than the static misalignment and casing deformation coefficient. The impulse shows a linear increase trend with the increasing static misalignment, and it decreases under the large n _p because the contact time decreases with the increase of n _p.     

A generalised model of milling forces

This paper presents the development of a generalised cutting force model for both end-milling and face-milling operations. The model specifies the interaction between workpiece and multiple cutter flutes by the convolution of cutting-edge geometry function with a train of impulses having the period equivalent to tooth spacing. Meanwhile, the effect of radial and axial depths of cut are represented by the modulation of the cutting-edge geometry function with a rectangular window function. This formulation leads to the development of an expression of end/face-milling forces in explicit terms of material properties, tool geometry, cutting parameters and process configuration. The explicitness of the resulting model provides a unique alternative to other studies in the literature commonly based on numerical integrations. The closed-form nature of the cutting force expression can facilitate the planning, optimisation, monitoring, and control of milling operations with complicated tool—work interactions. Experiments were performed over various cutting conditions and results are presented, in verification of the model fidelity, in both the angle and frequency domains.Notation * convolution operator - helix angle of an end mill - _A,_R axial and radial angles of a face mill - angular position of any cutting point in the cylindrical coordinate system - unit area impulse function - ^(i–1)(–T _o) (i–1)th derivative of (–T _o) with respect to - angular position of cutter in the negative Y-direction - _L, lead and inclination angles of a face mill - angular position of any cutting point in the negative Y-direction - ₁, ₂ entry and exit angles - upper limit of cutting edge function in terms of - as defined in equation (10) - A _xk ,A _yk ,A _zk kth harmonics of cutting forces in the X-, Y-, and Z-directions - d _a,d _r axial and radial depth of cut - dA instantaneous cut area - D diameter of cutter - f _o frequency of spindle - f _t,f _r,f _a local cutting forces in the tangential, radial, and axial directions - f _x ,f _y ,f _z local cutting forces in the X-, Y-, and Z-directions - F _x ,F _y ,F _z resultant cutting forces in the angle domain in the X-, Y-, and Z-directions - F as defined in equation (5) - h derivative of height function of cutting edge with respect to - h() height function of one cutting edge with respect to - H height of any cutting point - K _r,K _a radial-to-tangential and axial-to-tangential cutting force ratios - K _t tangential cutting pressure constant - K as defined in equation (6) - p as defined in equation (6) - N number of cutting edges - r() radius function of one cutting edge with respect to - R radius of any cutting point - T cutting engagement time function of any cutting point - T _o cutting engagement time of the cutting point at =0 - T th() tooth sequence function - t _c average cut thickness - t _x feed per tooth - W _A,W _W,W _C amplitude, width and centre of a window function - W(,) unit rectangular window function - y _min,y _max minimum and maximum positions of workpiece in the Y-direction - Z _min,Z _max integration limits in the Z-direction     

Experiments and theoretical approaches on the burning behaviors of single n-heptane droplet

This study was conducted to improve the theoretical prediction of the burning characteristics of an n-heptane droplet by comparing them with experimental results. To achieve this, numerical approaches were conducted by assuming that the droplet combustion can be described by both quasi-steady behavior for the region between the droplet surface and the flame interface, and transient behavior for the region between the flame interface and ambient surrounding. Comparisons were considered for droplet diameter (d _t ), flame diameter (d _f ), flame standoff ratio (FSR), and viscous drag induced fluxes which are Stefan flux and thermophoretic flux for various initial droplet diameter (d ₀ ) and oxygen (O ₂ ) concentration conditions. It was revealed that the flame diameter (d _f ) and flame standoff ratio (FSR) initially increase dramatically and approach quasi-steady behavior within the observation period, and the flame standoff ratio (FSR) increases a little with the initial droplet diameter (d ₀ ) both experimentally and theoretically. The value of flame diameter (d _f ) decreases from its maximum value when oxygen (O ₂ ) concentration is increased from a value of 18% to 40%. The burning rate (K) constant becomes higher as the oxygen (O ₂ ) concentration increases since the increase of oxygen (O ₂ ) concentration produces a higher maximum flame temperature (d _f ) which enhances the effective thermo-physical properties of the gas-phase bounded by droplet and flame front.

     

An Eddy-Current Study of Decay of Oversaturated Solid Solution in N32T3 Invar

The eddy-current parameter f ₀ has been studied as a function of temperature and aging time for the N32T3 Invar. Two initial states have been involved: those after quenching and after cold plastic deformation. The rise in the temperature and time of aging reduces the parameter f ₀ owing to the transition of the Invar from the ferromagnetic to paramagnetic state because the oversaturated solid solution decays, and the austenite is depleted of nickel. The parameter f ₀ drops monotonically with the applied strain. In the strained Invar the decay of the oversaturated solid solution reduces f ₀ in the temperature interval of 400 to 600°C.     

Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

Characterization of thermoelectric generators by measuring the load-dependence behavior

Solid-state thermoelectric generators (TEGs) based on the Seebeck effect to convert temperature gradients, ΔT [K], into electrical energy are being used in an increased number of stand-alone microsystems applications. These generators are composed by at least one pair of p- and n-type thermoelectric elements with high figures-of-merit, ZT, to perform such a conversion. The exact behavior knowledge of generators is mandatory in order to decide the most suitable for the target application. The focus of this paper is to present a methodology to characterize thermoelectric generators, by measuring their behavior for different types of loads. The measurements were done with the help of commercial thermoelectric generators (thermoelectric modules TEC1-12707) and a measurement setup composed by a controlled hot-plate, a controlled cooling fan (above an heat dissipator), a set of two thermistors for measuring the temperature, a personal computer with the data acquisition board model NI USB-6009 and the LabView software from National Instruments for acquiring the measures and for controlling both the hot-plate and the cooling fan. The thermoelectric modules TEC1-12707 was selected due to its compact size (e.g., 40 mm × 40 mm) and because it can withstand temperatures up to 450 K without degrading the quality of measurements. A SPICE model for thermoelectric modules TEC1-12707 was also obtained: an open-circuit voltage of V_open = 53.17 × ΔT [mV] and an internal resistance of R₀ = 3.88 Ω with a tolerance of ΔR_int = 0.13 Ω such that R_int = R₀ ± ΔR_int = 3.88 ± 0.13 Ω. The measurements done under the maximum output power delivery condition (for the maximum temperature gradient of ΔT = 51 °C) resulted in the maximum output power of P_out = 500 mW, as well as in the output current and voltage of I_out = 357 mA and V_out = 1.40 V, respectively. The load resistance of 3.92 Ω (V_out/I_out) is also in accordance with the measurements because it is located in the range [μ − σ, μ + σ] Ω, where μ = R₀ = 3.88 Ω and σ = ΔR_int = 0.13 Ω. An Agilent multimeter model 34410A with 6½ digits was used for measuring the voltages at the TEG’s output and the respective currents.