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NIM-Sr2 optical lattice clock has been developed on the Changping campus of National Institute of Metrology (NIM). Considering the limitations in NIM-Sr1, several improved parts have been designed including a differential pumping stage in the vacuum system, a permanent magnet Zeeman slower, water-cooled anti-Helmholtz coils, an extended viewport for Zeeman slower, etc. A clock laser with a short-time stability better than 3 × 10−16 is realized based on a self-designed 30-cm-long ultra-low expansion cavity. The systematic frequency shift has been evaluated to an uncertainty of 7.2 × 10−18, with the uncertainty of BBR shift and the collisional frequency shift being an order of magnitude lower than the last evaluation of NIM-Sr1.

Acquiring information of the neural structures in the whole-brain level is vital for systematically exploring mechanisms and principles of brain function and dysfunction. Most methods for whole brain imaging, while capable of capturing the complete morphology of neurons, usually involve complex sample preparation and several days of image acquisition. The whole process including optical clearing or resin embedding is time consuming for a quick survey of the distribution of specific neural circuits in the whole brain. Here, we develop a high-throughput light-sheet tomography platform (HLTP), which requires minimum sample preparation. This method does not require optical clearing for block face light sheet imaging. After fixation using paraformaldehyde, an aligned 3 dimensional image dataset of a whole mouse brain can be obtained within 5 hours at a voxel size of 1.30 × 1.30 × 0.92 μm. HLTP could be a very efficient tool for quick exploration and visualization of brain-wide distribution of specific neurons or neural circuits.

The cellular automaton model is suggested to describe the traffic-flow at the grade roundabout crossing. After the simulation with computer, the fundamental properties of this model have been revealed. Analysing this kind of road structure, this paper transforms the grade roundabout crossing with inner-roundabout-lane and outer-roundabout-lane into a configuration with many bottlenecks. Because of the self-organization, the traffic flow remains unblocked under a certain vehicle density. Some results of the simulation are close to the actual design parameter.

The harmonic optical frequency chain is the only tool for measuring optical frequency till the advent of a femtosecond optical frequency comb (FOFC). However, its disadvantages are obvious, such as high cost, difficult construction, complex usage, and complicated maintenance. The emergence of femtosecond optical frequency combs (FOFCs) makes it possible to measure the absolute frequency of a laser, which greatly simplifies the quantity traceability of the absolute frequency value and comparison, and allows the length unit “m” to be directly traced back to the time unit “s”. The beat note (fb) between an FOFC and a test laser is one of the most important data in measuring absolute frequency of the test laser. In order to ensure the accuracy and reliability of the measurement, the signal-to-noise ratio (SNR) of fb should be above 30 dB at 300 kHz resolution bandwidth. Among the wavelength standards recommended to replicate “meter” (SI), iodine-stabilized 633 nm lasers and iodine-stabilized 532 nm lasers have been widely used. Compared with iodine-stabilized 633 nm lasers, iodine-stabilized 532 nm lasers have the advantages of high stability, high output power, no modulation and fiber coupled output. Therefore, it is of great importance to measure and monitor the absolute frequency of an iodine-stabilized 532 nm laser. Aiming at the specific requirements for absolute frequency measurement of an iodine-stabilized 532 nm laser, the absolute frequency measurement of its fundamental 1064 nm laser has been studied. In this paper, a high-repetition-rate Er-doped femtosecond fiber laser is adopted as an optical source in the system. The repetition rate of the fiber laser is 303 MHz, the output power in the continuous-wave state is 130 mW and the average output power in the mode-locking state is 80 mW. The highest SNR of fb between the comb light and a 1064 nm laser generated by an iodine-stabilized 532 nm laser is only 30 dB due to the low intensity at 1 μm wavelength in the supercontinuum, which just reaches the SNR threshold meeting the counter's working condition. In order to improve the accuracy and reliability of absolute frequency measurement, the technique of cascading an Yb-doped fiber amplifier after spectral broadening is adopted to enhance the spectral intensity at 1 μm wavelength. The experimental results indicate that the SNR of fb between a 1 μm laser after spectral enhancement and a 1064 nm laser is increased by 5 dB and kept at 35 dB for several days, meeting requirements for long-term continuous monitoring. This method can effectively reduce the intensity requirements at 1 μm wavelength when the spectrum is directly broadened in the Er-FOFC.

An optical lattice clock based on 87Sr is built at National Institute of Metrology (NIM) of China. The systematic frequency shifts of the clock are evaluated with a total uncertainty of 2.3×10−16. To measure its absolute frequency with respect to NIM's cesium fountain clock NIM5, the frequency of a flywheel H-maser of NIM5 is transferred to the Sr laboratory through a 50-km-long fiber. A fiber optical frequency comb, phase-locked to the reference frequency of this H-maser, is used for the optical frequency measurement. The absolute frequency of this Sr clock is measured to be 429228004229873.7(1.4)Hz.

Femtosecond optical frequency combs (FOFCs) with output wavelengths covering visible light have potential applications in absolute frequency measureflent of iodine-stabilized lasers and optical clock lasers. Based on optical amplification, frequency doubling and spectrum broadening, a home-made Er-fiber femtosecond optical frequency comb (Er-FOFC) with output wavelengths covering visible light is demonstrated. One path with an average power of 8 mW from Er-FOFC is used as the seed pulse for spectrum broadening to cover the visible light. This path is first amplified to 532 mW by injecting into an Er-doped femtosecond fiber amplifier with combined forward and backward pumping and then frequency doubled with a MgO: PPLN crystal with an output power of 85 mW, frequency-doubling efficiency of 32% and pulse duration of 85fs. The output power of this path can be first amplified to 532 mW through an Er-doped femtosecond fiber amplifier when the forward pumping and backward pumping both turn on. Then the frequency-doubling laser can be generated in a MgO: PPLN crystal. The frequency-doubling efficiency is 32% and the pulse duration is 85 fs; the frequency-doubling light is spectrally broadened from 500 to 1000 nm in a photonic crystal fiber (PCF), with an output power of 85 mW and coupling efficiency of 50%. To verify the performance of the broadened spectrum, the light from the Er-FOFC and a compact iodine-stabilized frequency-doubled Nd: YAG laser at 532 nm is beaten. A beat signal with a signal-to-noise ratio of 30 dB at 100 kHz RBW is obtained, which provides a useful tool for absolute frequency measureflent of visible lasers.

High-repetition-rate fiber laser has widely applications in the field of femtosecond frequency comb, ultra-fast optical sampling, and so on. In this paper, an Er-doped femtosecond fiber laser with a repetition rate of 303 MHz is demonstrated based on the mechanism of nonlinear polarization rotation. By means of optimization of cavity dispersion, the net dispersion in fiber cavity is a little negative nearby zero point. After mode locking, the laser is operating in the stretched-pulse regime. At a pump power of 817 mW, the output power of the laser is 125 mW in continue-wave state, and 69 mW in mode-locking state. The laser directly generates 90 fs before dispersion compensation. Mode locking can self-start at the pump power of 700-817 mW. Repetition rate drift of the mode-locked laser is 30 Hz in five hours.

The repetition rate and the carrier-envolope offset frequency of a home-made Er-doped fiber comb are precisely phase-locked. The residual fluctuations of the stabilized repetition rate and the carrier-envolope offset frequency are 0.515 mHz and 93.13 mHz respectively

We report the experimental realization of strontium magneto-optical trap (MOT) operating on the intercombination transition 1S0−3P1 at 689 nm, namely red MOT. A 689 nm laser used for cooling and trapping is injection locked to a master laser, whose linewidth is narrowed to 150 Hz by locking to a high finesse optical reference cavity. 88Sr atoms pre-cooled and trapped by the broad 1S0−1P1 transition at 461 nm are transferred to the red MOT with the help of a time sequence controller. The transfer ratio is about 20% and the red MOT's temperature is estimated to be less than 20 μK by the time-of-flight (TOF) image analysis.

This paper presents a cellular automaton traffic flow model with an open boundary condition to describe the traffic flow at a roundabout crossing with an inner roundabout lane and an outer roundabout lane. The simulation results show that the boundary condition, bottlenecks and the self-organization affect the traffic flow at the roundabout crossing. Because of the effect of bottlenecks, jams easily appear on the inner roundabout lane. To improve the capacity of the roundabout system, proper values of the enter probability α and the out probability β can be chosen.

The traffic flow at the roundabout crossing with the inner roundabout lane and the outer roundabout lane is described by a cellular automaton traffic flow model with open boundary condition. Simulation results show that the boundary condition and bottlenecks affect the traffic flow at the roundabout crossing. The enter probability α and the out probability β can change the systemic flux in a certain range. Three phases of the system are differentiated by the relationship between the systemic flux and the enter probability α,the out probability β. The systemic phase diagram is also studied. The research can provide a reference for the realistic traffic control.

We report the experimental realization of a 88Sr magneto-optical trap (MOT) operating at the wavelength of 461 nm. The MOT is loaded via a 32 cm long spin-flip type Zeeman slower which enhances the MOT population by a factor of 22. The total laser power available in our experiment is about 300 mW. We have trapped 1.6 × 108 88Sr atoms with a 679nm and 707 nm repumping laser. The two repumping lasers enhance the trap population and trap lifetime by factors of 11 and 7, respectively. The 88Sr cloud has a temperature of about 2.3 mK, measured by recording the time evolution of the absorption signal.

In this paper，a cellular automaton model is proposed to describe the traffic flow at the grade roundabout crossing with the inner roundabout lane and the outer roundabout lane. Simulation results show that the traffic jam appears evidently at the intersection of the inner roundabout lane and the entering lane. The effect of the nonlinear reciprocity of the vehicle in the inner roundabout lane increases and the flux decreases with the increase of the deceleration probability. The flux of the system is improved by the increase of the turning probability.

In this paper we report on the Ti:sapphire mode-locked femtosecond laser based optical frequency comb apparatus and developed by NIM its application in optical frequency measurement of the I2 stabilized 532nm Nd∶YAG laser installed in NIM. The measurement result of 127I2 R(56)32-10 hyperfine transition is 563260223512991±20 (Hz), with a relative frequency uncertainty of 3.6×10-14. To our knowledge，this result is the first optical frequency measurement in China which is referenced directly to the Cs microwave frequency standard.

Using the improved Nagel-Schreckenberg traffic model with open boundary conditions,we present the fundamental diagrams of traffic flow under different parameters by numerical simulation.It turns out that the traffic flow of the improved Nagel-Schrec-kenberg model is greater than that of the Nagel-Schreckenberg model,then we analyse the characters and critical points of the transition from free flow phase to maximum current phase and the transition from jamming phase to free flow phase.