Anomalous Variation of Beat Frequency in a Dual Frequency He-Ne Laser

The beat frequency in a dual frequency He-Ne laser varies while the resonant cavity length is tuned. As to the laser with two longitudinal modes, the variation amplitude is commonly less than 500 kHz, proven by experiments and theories. This study reveals an anomalous variation of the beat frequency when a piece of element is put into the cavity and is aligned with the laser axis. Consequently the variation amplitude couM reach 22 MHz, several dozen times larger than that without the intra-cavity element. This cannot be explained only by laser mode pulling and pushing effects. Some influencing factors are investigated experimentally, including the tilted angle of the element and the distance between its surface and cavity mirror. The qualitative analysis is discussed, which agrees with the experimental results.

Temperature-sensing scheme based on a passively mode-locked fiber laser via beat frequency demodulation

In this paper,we propose a temperature-sensing scheme utilizing a passively mode-locked fiber laser combined with the beat frequency demodulation system.The erbium-doped fiber is used in the laser ring cavity to provide the gain and different lengths of single-mode fibers inserted into the fiber ring cavity operate as the sensing element.Different temperature sensitivities have been acquired in the experiment by monitoring the beat frequency signals at different frequencies.The experimental results indicate that the beat frequency shift has a good linear response to the temperature change.The sensitivity of the proposed sensor is about-44 kHz/℃ when the monitored beat frequency signal is about 10 GHz and the ratio of the sensing fiber to the overall length of the laser cavity is 10 m/17.5 m,while the signal-to-noise ratio(SNR)of the monitored signal is approximately 30 dB.The proposed temperature-sensing scheme enjoys attractive features such as tailorable high sensitivity,good reliability,high SNR,and low cost,and is considered to have great potential in practical sensing applications.

Kinematics Modeling and Experiments of Pectoral Oscillation Propulsion Robotic Fish

A robotic fish driven by oscillating fins, 'Cownose Ray-I', is developed, which is in dorsoventrally flattened shape without a tail. The robotic fish is composed of a body and two lateral fins. A three-factor kinematic model is established and used in the design of a mechanism. By controlling the three kinematic parameters, the robotic fish can accelerate and maneuver. Forward velocity is dependent on the largest amplitude and the number of waves in the fins, while the relative contribution of fin beat frequency to the forward velocity of the robotic fish is different from the usual result. On the other hand, experimental results on maneuvering show that phase difference has a stronger effect on swerving than the largest amplitude to some extent. In addition, as propulsion waves pass from the trailing edge to the leading edge, the robotic fish attains a backward velocity of 0. 15 m·s^(-1).

A novel orthogonally linearly polarized Nd:YVO_4 laser

We presented a novel orthogonally linearly polarized Nd：YVO4 laser. Two pieces of a-cut grown-together composite YVO4/Nd：YVO4 crystals were placed in the resonant cavity with the c-axis of the two crystals orthogonally. The polarization and power performance of the orthogonally polarized laser were investigated. A 26.2-W orthogonally linearly polarized laser was obtained. The power ratio between the two orthogonally polarized lasers was varied with the pump power caused by the polarized mode coupling. The longitudinal modes competition and the corresponding variable optical beats were also observed from the orthogonally polarized laser. We also adjusted the crystals with their c-axis parallele to each other, and a 40.7-W linearly polarized TEM00 laser was obtained, and the beam quality factors were Mx^2 = 1.37 and My^2 = 1.25.

Optical beat notes assisted attosecond soft X-ray pulse generation in high-gain free electron lasers

Attosecond soft X-ray pulses are of great importance for the study of ultrafast electronic phenomena.In this paper,a feasible method is proposed to generate isolated fully coherent attosecond soft X-ray free electron laser via optical frequency beating.Two optical lasers with the opposite frequency chirps are used to induce a gradient frequency energy modulation,which helps to generate a gradually varied spacing electron pulse train.Subsequently,the undulator sections with electron beam delay lines are used to amplify the target ultra-short radiation.Numerical start-to-end simulations have been performed and the results demonstrate that an isolated soft X-ray pulse with the peak power of 330 GW and pulse duration of 620 as can be achieved by the proposed technique.

Michelson interferometer composite cavity fiber laser sensor with radio frequency interrogation

A Michelson interferometer(MI) composite cavity fiber laser sensing system based on radio frequency(RF) interrogation is proposed and experimentally demonstrated. The system down-converts the traditional MI light frequency detection to RF detection, which improves the stability of the system. The optic fiber MI is placed in the laser resonator to form a composite cavity structure, which greatly improves the sensitivity of beat frequency signal demodulation.

A Heterodyne Interferometer with Separated Beam Paths for High-Precision Displacement and Angular Measurements

As standard concepts for precision positioning within a machine reach their limits with increasing measurement volumes,inverse concepts are a promising approach for addressing this problem.The inverse principle entails other limitations,as for high-precision positioning of a sensor head within a large measurement volume,three four-beam interferometers are required in order to measure all necessary translations and rotations of the sensor head and reconstruct the topography of the reference system consisting of fixed mirrors in the x-,y-,and z-directions.We present the principle of a passive heterodyne laser interferometer with consequently separated beam paths for the individual heterodyne frequencies.The beam path design is illustrated and described,as well as the design of the signal-processing and evaluation algorithm,which is implemented using a System-On-a-Chip with an integrated FPGA,CPU,and A/D converters.A streamlined bench-top optical assembly was set up and measurements were carried out to investigate the remaining non-linearities.Additionally,reference measurements with a commercial homodyne interferometer were executed.

Comparison of human nasal epithelial cells grown as explant outgrowth cultures or dissociated tissue cultures in vitro

The purpose of this study was to compare cell growth characteristics,ciliated cell differentiation,and function of human nasal epithelial cells established as explant outgrowth cultures or dissociated tissue cultures.Human nasal mucosa of the uncinate process was obtained by endoscopy and epithelial cell cultures were established by explant outgrowth or dissociated tissue culture methods.Epithelial cell growth characteristics were observed by inverted phase contrast microscopy.Ciliated cell differentiation was detected byβ-tubulin IV and ZO-1 immunocytochemistry.Basal and ATP-stimulated ciliary beat frequency(CBF)was measured using a high-speed digital microscopic imaging system.Both the explant and dissociated tissue cultures established as monolayers with tight junctions and differentiated cell composition,with both types of cultures comprising ciliated and non-ciliated epithelial cells.Fibroblasts were also frequently found in explant cultures but rarely seen in dissociated tissue cultures.In both culture systems,the highest ciliated cell density appeared at 7th–10th culture day and declined with time,with the lifespan of ciliated cells ranging from 14 to 21 days.Overall,10%of the cells in explant cultures and 20%of the cells in the dissociated tissue cultures were ciliated.These two cultures demonstrated similar ciliary beat frequency values at baseline(7.78±1.99 Hz and 7.91±2.52 Hz,respectively)and reacted equivalently following stimulation with 100μM ATP.The results of this study indicate that both the explant outgrowth and dissociated tissue culture techniques are suitable for growing well-differentiated nasal ciliated and non-ciliated cells,which have growth characteristics and ciliary activity similar to those of nasal epithelial cells in vivo.

Solid-state transformer-based new traction drive system and control

A new type of traction drive system consisting of solid-state traction transformer （SSTT）, inverter unit, auxiliary inverter, traction motor and other key components is built in order to suit the demand of developing the next-generation electric traction system which will be efficient and lightweight, with high power density. For the purpose of reducing system volume and weight and improving efficiency and grid-side power quality, an efficient SSTT optimized topology combining highvoltage cascaded rectifiers with high-power high-frequency LLC resonant converter is proposed. On this basis, an integrated control strategy built upon synchronous rotating reference frame is presented to achieve unified control over fundamental active, reactive and harmonic components. The cartier-interleaving phase shift modulation strategy is proposed to improve the harmonic performance of cascaded rectifiers. In view of the secondary pulsating existing in a single-phase system, the mathematical model of secondary power transfer is built, and the mechanism of pulsating voltage resulting in beat frequency of LLC resonant converter is revealed, so as to design optimum matching of system parameters. Simulation and experimental results have verified that the traction system and control scheme mentioned in this paper are reasonable and superior and that they meet the future application requirements for rail transit.