No abstract

A variety of technologies have been so far demonstrated for the carrier-envelope phase (CEP) control of mode-locked oscillators. An instrumental part of these technologies is the configuration of interferometers and the application of feedback control. Of the devices, the collinear ƒ-2ƒ interferometer developed in this work is compact and robust against disturbance because the optical paths of the ƒ and 2 ƒ components are collinear within the interferometer. To compensate for the delay time between the ƒ and 2ƒ components, a birefringent time plate made of α-BBO is installed in the interferometer. We achieved an in-loop CEP stability of 27 mrad (rms) using conventional feedback control with an acousto-optic modulator to control the pump power. We believe that the collinear ƒ-2ƒ interferometer offers the best choice as an interferometer for CEP stabilization of the front-end oscillator in the chirped-pulse amplification system

To generate an intense femtosecond pulse with an energy of several mJ or more, the pulse must be sufficiently stretched and compressed using grating-based optics in a chirped pulse amplification system. However, the stability of the carrier–envelope phase (CEP) is readily degraded by movement of the diffraction gratings due to vibration caused by pump lasers and other equipment. By suppressing the vibration and acoustic waves, we reduced the CEP noise to much less than 100 mrad (rms). With feedback control using a wedge pair with a single shot f - 2 f interferometer (1 ms integration time), we were able to stabilize the CEP, achieving a CEP noise of 58 mrad (rms) at a pulse energy of 0.5 mJ and 99 mrad (rms) at a pulse energy of 7 mJ and a repetition rate of 1 kHz. We also analyzed the effects of vibration on the pulse stretcher and compressor using a geometrical optics approach and found that roof mirrors were the most influential.

This work is concerned with a technique to alleviate thermal concentration on specific switching devices that drive a permanent magnet synchronous motor (PMSM) under zero-speed and high-torque condition. In this condition, e.g., start or stop of an elevator or hill-start of an electric vehicle, a large DC current flows in the PMSM, and the heat generated in the specific switching devices is locally concentrated. The proposed technique uses a zero-sequence voltage in a three-level inverter, and the polarity of the zero-sequence voltage is switched according to the magnetic pole position of the PMSM. The proposed technique can change the current paths in the inverter, and the loss concentrations in specific devices can be alleviated. The simulation results show that the amplitude and the time ratio of the zero-sequence voltage that depend on the magnetic pole position of the PMSM affect the temperature rise of the power device with the maximum temperature. In the experiment, the effectiveness of the proposed technique is evaluated using a small power inverter. This three-level inverter consists of discrete power devices so that the surface temperature of each device can be observed with a thermal camera. The experimental results show that the temperature rise of the device with the maximum temperature is reduced by about 31%.