Cold atom clocks and their applications in precision measurements

Cold atom clocks have made remarkable progresses in the last two decades and played critical roles in precision measurements. Primary Cs fountain frequency standards have achieved a total uncertainty of a few parts in 1016, and the best optical clock has reached a type B uncertainty below 10-18. Besides applications in the metrology, navigation, etc.,ultra-stable and ultra-accurate atomic clocks have also become powerful tools in the basic scientific investigations. In this paper, we focus on the recent developments in the high-performance cold atomic clocks which can be used as frequency standards to calibrate atomic time scales. The basic principles, performances, and limitations of fountain clocks and optical clocks based on signal trapped ion or neutral atoms are summarized. Their applications in metrology and other areas are briefly introduced.

A Novel Blood Coagulation Measuring Method Based on the Viscoelasticity of Non-Newtonian Fluid

There are complex and perfect coagulation, anticoagulation and fibrinolysis systems in the human body and their fine regulatory mechanisms. Once the coagulation system and its regulatory mechanisms are destroyed, bleeding or thrombosis will occur very soon. In the blood coagulation, the blood viscoelasticity changes. Therefore, the thrombus elasticity measurement technology can be used to continuously monitor the changing blood viscoelasticity in order to study the process of coagulation. The results of the interaction among the various components of the blood can be obtained from coagulation to fibrinolysis by bedside detection. The traditional electromagnetic induction sensors, based on conventional coil inductance, are manufactured complexly, high cost and non-linear. Therefore, this paper proposes a non-Newtonian fluid viscoelasticity measurement method based on the piezoelectric effect. We use the piezoelectric bimorphs with the diameter of 21 mm and the total thickness of 0.38 mm and DSM coupling probes with the length of 3 mm, 5 mm and 7 mm to design the piezoelectric bimorphs driver. The viscoelasticity of different non-Newtonian fluids is tested. The vibration amplitudes of the piezoelectric bimorphs and liquid surfaces range from 0.43 μm to 3.52 μm. Consequently, the feasibility of in vitro detection of thrombus is confirmed in principle and the above scheme is validated theoretically and experimentally, which provides the basis for the measurement of blood viscoelasticity, the in vitro detection of thrombus and the manufacture of blood coagulation instrument.

Precision Controlling of Frequency Difference for Elastic-Stress Birefringence He-Ne Dual-Frequency Lasers

Birefringence-Zeeman dual frequency lasers are capable of producing frequency difference from several kilohertz to hundreds of megahertz, but the precision of giving and stabilizing of the beat frequency still needs improvement to the range of ±200 kHz. We design a new elastic force-exerting device comprised of the bottom part, two arms and two pieces of force-exerting sheets. The frequency difference smoothly tuning is realized with this device in a large range of 2 MHz to 20 MHz. Power-balance frequency stabilization system is used to investigate characters of the temperature, frequency difference and laser power. The precision of the frequency difference has reach up to ±100 kHz after system temperature balance. Analyses of the laser frequency difference and power character are carried out.

Laser Feedback Technique for Precise Retardation Measurements

A simple and precise retardation measurement based on laser feedback is demonstrated. The measurement principle is based on polarization flipping induced by optical feedback from an external birefringence cavity. The measured wave plate is located in the external cavity. When the length of the external cavity is tuned, the polarization states of laser will flip between two eigenstates, and the position of polarization flipping in one period of intensity modulation will vary with retardation of the wave plate. The duty ratio of two eigenstates is used to determine the retardation. Main advantages of the technique are that it is compact, low cost, fast and flexible. Especially, it is insensitive to a fluctuation of laser intensity and is suitable for on-line measurement. The experimental results have shown that the measurement uncertainty is better than 0.03° in the range 30°-150°.

Intensity modulation characters of orthogonally polarized HeNe lasers with different optical feedback level

This paper demonstrates the intensity modulation characters of orthogonally polarized HeNe lasers with different optical feedback level generated by the variable reflectivity of external reflector. The modulation depths of the orthogonally polarized frequencies are increased when the optical feedback level becomes strong. It also observes that the modulation amplitudes are different for different external cavity length. Based on the vectorial extension of Lamb＇s semi-classical theory, it finds that the calculations are consistent with the experimental results.

Influence of Feedback Levels on Polarized Optical Feedback Characteristics in Zeeman-Birefringence Dual Frequency Lasers

紧张和极化 propertiesof 上的反馈层次的影响极化了在 Zeeman-birefringence 双频率激光的光反馈是 systematicallyinvestigated。由改变反馈力量比率，不同反馈层次被获得。极化的光反馈效果的 Threedistinct 政体被发现并且定义为政体我， II andIII。在政体之中的反馈水平边界试验性地被获得。theoreticalanalysis 被介绍在对试验性的结果的好同意。

Anisotropic optical feedback of single frequency intra-cavity He-Ne laser

This paper presents the anisotropic optical feedback of a single frequency intra-cavity He-Ne laser. A novel phenomenon was discovered that the laser output an elliptical polarized frequency instead of the initial linear polarized one. Two intensities with a phase difference were detected, both of which were modulated in the form of cosine wave and a fringe shift corresponds to a λ/2 movement of the feedback mirror. The phase difference can be continuously modulated by the wave plate in the external cavity. Frequency stabilization was used to stabilize the laser frequency so as to enlarge the measuring range and improve the measurement precision. This anisotropic optical feedback system offers a potential displacement measurement technology with the function of subdivision of λ/2 and in-time direction judgment. The three-mirror Fabry Perot cavity model is used to present the experimental results. Given the lack of need of lasing adjustment, this full intra-cavity laser can significantly improve the simplicity and stability of the optical feedback system.

Self Mixing Fringes of Vertical-Cavity Surface-Emitting Lasers under Dual Reflector Feedback

由于目标的每二十分之一个波长排水量变的自我混合的穗被观察。在激光的外部洞利用双反射镜，传感器的决定到 10 次被改进了。每个反射镜的角色详细被讨论了。

Intensity tuning characteristics of double-mode He-Ne laser with optical feedback

This paper investigates the intensity tuning characteristics of a double longitudinal modes HeiNe laser subjected to optical feedback. The intensity undulations of the total light and the two modes are observed for different external cavity length. Two modulations of the internal cavity length are performed. One is only for the internal cavity length being modulated and the other is for both the internal and the external cavity length being modulated. The undulation frequency of the total light is found to be determined by the ratio of external cavity length to internal cavity length in both modulations. When the external cavity length is integral times of the internal cavity length, the fringe frequency of the total light could be seven or even more times of that in conventional optical feedback. A simple theoretical analysis is presented, which is in good agreement with the experimental results. The potential use of the experimental results is also discussed.

Influence of Feedback Level on Laser Polarization in Polarized Optical Feedback

The influence of feedback level(or the amplitude of feedback light)on laser polarization in polarized optical feedback is investigated.A polarizer is placed in the feedback cavity to form the polarized feedback,and an attenuator is placed in the feedback cavity to tune the feedback level.The laser intensity and polarization vary dramatically at different transmissivities of the attenuator.According to the experimental phenomenon,the range of the attenuator transmissivity is divided into three zones:the flipping zone,monostable zone,and bistable zone.In the monostable zone,the laser polarization is always perpendicular to the axis of the polarizer in the feedback cavity.This may provide an effective way to choose or control laser polarization.A theoretical model based on the self-consistency of laser oscillation is presented to analyze the experimental results.

Distortion of optical feedback signals in microchip Nd:YAG lasers subjected to external multi-beam interference feedback

This paper proposes a theoretical analysis for the characteristics of an external cavity Nd：YAG laser with feedback of multiple-beam interference, which is induced by the multi-reentrance of the light from the external Fabry-Perot cavity. The theoretical model considers the multiple beam interference of the external Fabry-Perot cavity. It is found that the optical feedback signals are distorted to pulse waveforms instead of the sinusoidal ones in conventional feedback. The experimental results are in good agreement with the theoretical analysis. The obtained theoretical and experimental results can advance the development of a laser feedback interferometer.

Electrochemical performance of a three-layer electrode based on Bi nanoparticles,multi-walled carbon nanotube composites for simultaneous Hg(Ⅱ)and Cu(Ⅱ)detection

Electrochemical analysis is a promising technique for detecting biotoxic and non-biodegradable heavy metals.This article proposes a novel composite electrode based on a polyaniline(PANi)fra mework doped with bismuth nanoparticle@graphene oxide multi-walled carbon nano tubes(Bi NPs@GO-MWCNTs)for the simultaneous detection of multiple heavy metal ions.Composite electrodes are prepared on screenprinted electrodes(SPCEs)using an efficient dispensing technique.We used a SM200 SX-3 A dispenser to load a laborato ry-specific ink with optimized viscosity and adhesion to draw a pattern on the work area.The SPCE was used as substrate to facilitate cost-effective and more convenient real-time detection technology.Electrochemical techniques,such as cyclic voltammetry and differential pulse voltammetry,were used to demonstrate the sensing capabilities of the proposed sensor,The sensitivity,limit of detection,and linear range of the PANi-Bi NPs@GO-MWCNT electrode are 2.57×10^(2)μA Lμmol^(-1) cm^-2,0.01 nmol/L,and 0.01 nmol/L-5 mmol/L and 0.15×10^(-1)μALμmol^(-1) cm^-2,0.5 nmol/L,and 0.5 nmol/L-5 mmol/L for mercury ion(Hg(Ⅱ))and copper ion(Cu(Ⅱ))detection,respectively.In addition,the electrode exhibits a good selectivity and repeatability for Hg(Ⅱ)and Cu(Ⅱ)sensing when tested in a complex heavy metal ion solution.The constructed electrode system exhibits a detection performance superior to similar methods and also increases the types of heavy metal ions that can be detected.Therefore,the proposed device can be used as an efficient sensor for the detection of multiple heavy metal ions in complex environments.

Intensity Tuning in Single Mode Microchip Nd：YAG Laser with External Cavity

我们调查紧张在单个模式微芯片 Nd 调节的特征: 有一个外部洞的钇铝柘榴石激光。在内部洞长度的一个时期 oflambda/2 变化的激光紧张的波动被观察。二个不同光反馈案例被执行。一个人是完美地排列的一个外部洞反射镜，其它是倾斜的一个外部洞反射镜。然而，在在这二个案例中的内部洞长度的 lambda/2change 的一个时期的激光紧张的变化频率被发现被外部洞长度的比率决定到内部洞长度。同时为倾斜的外部洞，变化频率也与在倾斜的外部洞的多重反馈有关。

Optical Feedback Characteristics in He-Ne Dual Frequency Lasers

在 He-Ne 双频率激光的光反馈特征与一个可变衰减器在不同反馈力量比率系统地被学习。反馈力量比率从 0.010 变化直到 0.998。自我干扰效果的五不同政体被发现并且定义为政体我， II， III， IV 和 V。因此，五个光反馈层次在 He-Ne 双频率激光被提出了。强壮的模式比赛在政体 III 和 IV.In 政体 V 被观察，多重反馈效果被调查。基本理论分析也被介绍。我们的结果能预付自我混合的干涉仪和 He-Ne 的排水量传感器的研究直角地极化的双频率激光。

Strong optical feedback in birefringent dual frequency laser

Strong optical feedback in a birefringent dual frequency He-Ne laser with a high reflectivity feedback mirror has been investigated for the first time. The output characteristics of two orthogonally polarized modes are demonstrated in two different optical feedback cases： one is for both modes being fed back and the other is for only one of the modes being fed back. Strong mode competition can be observed between the two modes with strong optical feedback, And when one mode＇s intensity is near its maximum, the other mode is nearly extinguished. When both modes are fed back into the laser cavity, the mode competition is stronger than when only one mode is fed back, The difference in initial intensity between the two orthogonally polarized modes plays an important role in the mode competition, which has been experimentally and theoretically demonstrated.

Mode hopping in single-mode microchip Nd：YAG lasers induced by optical feedback

The mode hopping phenomenon induced by optical feedback in single-mode microchip Nd：YAG lasers is presented. With optical feedback, mode hopping strongly depends on two factors： the ratio of external cavity length to intra-cavity length, and initial gains of the two hopping modes, When external cavity length equals an integral multiple of intracavity length, there is almost no mode hopping. However, if the external cavity length does not equal an integral multiple of intra-cavity length, mode hopping occurs. The ratio of external cavity length to intra-cavity length determines the position of two-mode hopping, The initial gains of the two hopping modes determine the corresponding peak values and oscillating periods of them in the intensity modulation curves.

Controlling the surface buckling wrinkles by patterning the material system of hard-nano-film/soft-matter-substrate

Wrinkling and buckling of nano-films on the compliant substrate are always induced due to thermal deformation mismatch.This paper proposes effective means to control the surface wrinkling of thin film on the compliant substrate,which exploits the curvatures of the curve cracks designed on the stiff film.The procedures of the method are summarized as:1)curve patterns are fabricated on the surface of PDMS(Polydimethylsiloxane)substrate and then the aluminum film with the thickness of several hundred nano-meters is deposited on the substrate;2)the curve patterns are transferred onto the aluminum film and lead to cracking of the film along the curves.The cracking redistributes the stress in the compressed film on the substrate;3)on the concave side of the curve,the wrinkling of the film surface is suppressed to be identified as shielding effect and on the convex side the wrinkling of the film surface is induced to be identified as inductive effect.The shielding and inductive effects make the dis-ordered wrinkling and buckling controllable.This phenomenon provides a potential application in the fabrication of flexible electronic devices.

Optical feedback characteristics in a helium neon laser with a birefringent internal cavity

The output characteristics of optical feedback in a helium neon laser with a birefringent internal cavity are studied systematically in five different regions of the gain curve for the two orthogonally polarized modes. When the laser operates in the two end regions of the laser gain curve, one of the two orthogonally polarized modes will be a leading one in optical feedback. Strong mode competition can be observed. However, when the laser operates in the middle region of the laser gain curve, the two modes can oscillate equally with optical feedback. Besides the intensity of the two polarized lights, the total light intensity is also studied at the same time. M-shaped optical feedback curves are found. Particularly, when the average intensities of the two lights are comparable, the intensity modulation curve of the total light is doubled, which can be used to improve the resolution of an optical feedback system.

Birefringence Optical Feedback with a Folded Cavity in HeNe Laser

The birefringence optical feedback with a folded cavity in HeNe laser is investigated.A theory model based on the equivalent cavity of the Fabry–Perot interferometer is presented.The phase difference between the two intensities in birefringence feedback is twice the retardation of the wave plate.The phase difference is invariable when the length of the feedback cavity changes.With the adoption of a cube corner prism(CCP)to form a folded cavity,the fringe frequency is doubled,and the resolution of the displacement sensor based on birefringence optical feedback with a folded cavity is improved.A resistance chain of 5-fold subdivision and 4-fold logic subdivision is used as further subdivision.The resolution ofλ/80 is obtained eventually;for 632.8 nm HeNe laser it is 7.91 nm.The displacement sensor based on birefringence optical feedback with a folded cavity is simple and of high resolution,large measurement range,low cost,and is of great application potential in industry.

A paper-based self-inductive folding displacement sensor for human respiration and motion signals measurement

Flexible sensors have recently attracted much attention for potential applications in motion measurement and wearable health monitoring.In this paper,an environment-friendly,economic,and high-performance paper-based self-inductive folding displacement sensor(PSIFS)was proposed,with displacement resolution of 20μm and wide measurement range of 43.2 mm.The sensor sensitivity could reach 4.44%mm^(−1) with the maximum deviation of 0.00904 in experiments.The sensing mechanism based on inductance variation due to three-dimensional deformation of planar inductor coil and structural design principle under two dimensions of sensitivity and size were proposed and studied.The folding method to enhance sensitivity was proposed and 3 times sensitivity enhancement could be achieved by once folding.Multiple breathing states and motion states of the human joints could be recognized.The proposed simple paper-based sensor,folding method to increase sensitivity,and structural design model may provide a way for the research of flexible sensors.