Arbitrary amplitude femtosecond pulse shaping via a digital micromirror device

An ultrafast spectrum programmable femtosecond laser may enhance the performance of a wide variety of scientific applications,e.g.,multi-photon imaging.In this paper,we report a digital micromirror device(DMD)-based ultrafast pulse shaper,i.e.,DUPS,for femtosecond laser arbitrary amplitude shaping-the first time a programmable binary device reported to shape the amplitudes of ultrafast pulses spectrum at up to 32 kHz rate over a broad wavelength range.The DUPS is highly effcient,compact,and low cost based on the use of a DMD in combination with a transmission grating.Spatial and temporal dispersion introduced by the DUPS is compensated by a quasi-4-f setup and a grating pair,respectively.Femtosecond pulses with arbitrary spectrum shapes,including rectangular,sawtooth,triangular,double-pulse,and exponential profile,have been demonstrated in our experiments.A feedback operation process is implemented in the DUPS to ensure a robust and repeatable shaping process.The total effciency of the DUPS for amplitude shaping is measured to be 27%.

Noise behaviors of a closed-loop micro-electromechanical system capacitive accelerometer

The noise of closed loop micro-electromechanical systems(MEMS) capacitive accelerometer is treated as one of the significant performance specifications.Traditional optimization of noise performance often focuses on designing large capacitive sensitivity accelerometer and applying closed loop structure to shape total noise,but different noise sources in closed loop and their behaviors at low frequencies are seldom carefully studied,especially their behaviors with different electronic parameters.In this work,a thorough noise analysis is established focusing on the four noise sources transfer functions near 0 Hz with simplified electronic parameters in closed loop,and it is found that the total electronic noise equivalent acceleration varies differently at different frequency points,such that the noise spectrum shape at low frequencies can be altered from 1/f noise-like shape to flat spectrum shape.The bias instability changes as a consequence.With appropriate parameters settings,the 670 Hz resonant frequency accelerometer can reach resolution of 2.6 μg/(Hz)1/2 at 2 Hz and 6 μg bias instability,and 1300 Hz accelerometer can achieve 5μg/(Hz)1/2 at 2 Hz and 31 μg bias instability.Both accelerometers have flat spectrum profile from 2 Hz to 15 Hz.

Precoding with interference suppression sequence scheme for OFDM-based cognitive radio systems

Orthogonal frequency division multiplexing （OFDM） is considered to be one of the promising schemes for cognitive radios （CR） systems, due to its flexibility in spectrum shaping. However, the traditional way of spectrum-notching failed to suppress out-of-band leakage enough, resulting in interference with licensed users （LUs）. A novel spectrum shaping scheme is proposed in this article, where a precoder with interference suppression sequence （ISS） is for the first time introduced for shaping the spectrum of OFDM signals in OFDM-based CR systems. The precoder matrix together with ISS matrix is obtained by minimizing the power of out-of-band interference, which results in the sufficient interference suppression of OFDM signal to LUs Analysis and simulation results show that the proposed scheme can significantly suppress the interference power to LUs at a little cost of precoding efficiency. Meanwhile, the peak-to-average-power-ratio （PAPR） is invariable and the bit-error-rate （BER） performance over Rayleigh fading channel is improved.