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This paper presents a 13-level switched capacitor inverter with a novel modulation method designed to minimize the number of switches and significantly reduce switching losses. The inverter stands out for its simplicity, requiring only ten semiconductor switches to generate 13 levels. A key feature is the inherent self-voltage balancing of the capacitors, which eliminates the need for additional control mechanisms. The inverter's unique architecture, comprising high-voltage and low-voltage modules, enables modulation using a hybrid PWM approach that combines step waveform modulation with level-shifted PWM (LS-PWM). This innovative technique dramatically lowers the switching frequency of the high-voltage module's switches, independent of the carrier frequency, thereby limiting the number of switches in high-frequency operation and achieving substantial reductions in switching losses. This paper provides a detailed analysis of the inverter's operating modes, voltage-balancing mechanisms, and parameter calculations. The advantages of the topology presented in this paper are demonstrated by comparison. Finally, the simulation and experimental results confirm the practicality and effectiveness of the proposed inverter and its modulation strategy. [ABSTRACT FROM AUTHOR]

A new Rb fountain clock was built at the National Institute of Metrology with the aim to realize robustness and excellent long-term instability for time keeping. While some basic designs have been adopted from our Cs fountain clocks, new features have been included for improved performance. A double-metal interrogation microwave cavity with a thermal expansion self-compensating mechanism was used to reduce the clock sensitivity to temperature fluctuations for the first time, to our knowledge. Moreover, a compact optical system was developed to ensure robustness. These features dramatically increased the ambient temperature range. The developed Rb fountain clock achieved a typical fractional frequency instability of 1.49×10−13(τ/s)−1/2within 10 days of continuous operation without any intervention, demonstrating the potential for time keeping.

We report an optical-scanning, dual-fiber, extrinsic Fabry-Perot interferometer system for absolute measurement of microdisplacement. The system involves two air-gapped Fabry-Perot cavities, formed by fiber end faces, functioning as sensing and reference elements. Taking the scanning wavelength as an interconverter to compare the gap length of the sensing head with the reference-cavity length yields the absolute measurement of the sensing-cavity length. The measurement is independent of the wavelength-scanning accuracy, and the reference-cavity length can be self-calibrated simply by one's changing the sensing-head length by an accurate value.

The paper describes a Raman-laser system with high performance for an absolute gravimeter that was based on 87Rb atom interferometer. As our gravimeter is a part of the standard acceleration of gravity of China, the Raman lasers' characteristics should be considered. This laser system includes two diode lasers. The master laser is frequency locked through the frequency-modulation (FM) spectroscopy technology. Its maximum frequency drift is better than 50 kHz in 11 h, which is measured by home-made optical frequency comb. The slave laser is phase locked to the master laser with a frequency difference of 6.8346 GHz while using an optical phase lock loop (OPLL). The phase noise is lower than −105 dBc/Hz at the Fourier frequency from 200 Hz to 42 kHz. It is limited by the measurement sensitivity of the signal source analyzer in low Fourier frequency. Furthermore, the power fluctuation of Raman lasers' pulses is also suppressed by a fast power servo system. While using this servo system, Raman lasers' pulses could be fast re-locked while its fast turning on again in the pulse sequence. The peak value fluctuation of the laser power pulses is decreased from 25% to 0.7%, which is improved over 35 times. This Raman-laser system can stably operate over 500 h, which is suited for long-term highly precise and accurate gravity measurements. [ABSTRACT FROM AUTHOR]