In recent years, dual repetition-rate mode-locked lasers with slightly different pulse repetition rates, as newly developed ultrafast lasers, have attracted great interest and shown their applications in ultrafast dual-comb spectroscopy, asynchronous optical sampling without mechanical movement, etc. The traditional dual-comb system composed of a pair of independent optical frequency combs with slightly detuned comb spacing is still considered expensive, complex and fragile. It is imperative to develop practical and compact dual-comb devices. Dual repetition-rate ultrafast lasers generating asynchronous ultrafast pulses directly from a single cavity can be a promising alternative to the current dual-laser-based comb source. A dual-comb setup based on single laser has the advantages of compact structure, low cost and intrinsic mutual coherence. This technique paves the way for developing the compact, robust and environmental-immune dual-comb systems. In this paper we develop an alternative dual repetition-rate mode-locked Yb:YAG ceramic laser that emits a pair of pulses with spatially separated beams from a single cavity by using a semiconductor saturable absorber mirror and a dual-path pump configuration. In our experiment, a high quality transparent Yb:YAG ceramic prepared by non-aqueous taper-casting method is selected as the gain medium, which is pumped by a 940 nm laser diode. A dual-path pump configuration consisting of a pair of polarization beam splitters and a pair of half-wave plates is designed, in which total pump power from a laser diode is divided equally for pumping the two separate laser beams. When the total absorbed pump power is 5.6 W, dual repetition-rate continuous mode-locked laser operation is achieved under the gain-loss balanced cavity condition. The pulse repetition rates of Pulse1 and Pulse2 are 448.918 MHz and 448.923 MHz, respectively. The difference between repetition rates is 5 kHz mainly caused by the different optical path lengths in the cavity. Under an absorbed pump power of 7 W, the maximum total output power extracted from this laser reaches 170 mW, i.e., 89 mW for Pulse1 and 81 mW for Pulse2. The two mode-locked pulses have nearly identical spectral shapes centered at 1029.6 nm and 1029.8 nm, respectively. The spectral bandwidths for Pulse1 and Pulse2 are 1 nm and 1.16 nm, respectively. The corresponding pulse durations are 2.8 ps and 2.6 ps for the Pulse1 and Pulse2 respectively. Our scheme integrates the advantages of self-starting operation, high repetition-rate, suppression of gain competition. These results indicate that dual-path pump configuration is feasible for dual-repetition-rate mode-locked lasers. These co-generated, dual repetition-rate pulses from one laser cavity possess similar laser characteristics and can be operated independently by dual-path pump configuration. This laser has potential advantages of compact, cost-effective and high-stability for single-cavity-based dual-comb applications in dual-comb spectroscopy, distance ranging, etc.
The properties of electromagnetic waves in atmospheric pressure plasmas are investigated with the Lorentz model. The effects of the electromagnetic wave frequency, plasma density, and the electron-neutral collision frequency on the attenuation of electromagnetic waves are discussed. The numerical results indicate that the wave attenuation is stronger at the region of the longer wavelength and at the higher plasma density.
A nonlinear pre-shaper which optimizes initial pulses for self-similar evolution in a following short fiber amplifier is demonstrated. It consists of a pair of transmission gratings and a segment of single mode fiber, by which pulses are shaped temporally and spectrally before amplification. To confirm the benefit of nonlinear pre-shaping for the self-similar evolution, pulse amplifications with and without the nonlinear pre-shaper are simulated. From comparison, pulses optimized by nonlinear pre-shaper show a shorter pulse duration, less pedestal and broader spectrum after amplification and compensation. With this optimization, the self-similar amplification can be realized in a 2.2-meter Yb3+-doped fiber in a large range of pump power, generating 60 fs transform-limited pulses after compression. This nonlinear pre-shaping method can efficiently shorten the fiber length and release the seed quality required for self-similar amplification. An all-normal dispersion mode-locked fiber laser is employed as the seed of a self-similar amplifier for the first time, thus facilitating an all-fiber system.
A femtosecond laser source tuned from red to mid-infrared is demonstrated. It is based on intracavity sum frequency generation of a MgO-doped periodically poled LiNbO3 optical parametric oscillator synchronously pumped by mode-locked Yb large-mode-area photonic crystal fiber, which has high average power and high repetition rate. The optical parametric oscillator has a wide spectral tuning range from 1450–2200 nm (for the signal) and 2250–4000 nm (for the idler) while the wavelength of the pump is 1040 nm. In the experiment, the output power of 374 mW at 1502 nm is achieved when the pump power is 2 W and the slope efficiency is 18.7%. In addition, 166 mW idler at 3.4 μm are achieved. By using a β-BaB2O4 for intracavity sum frequency generation, the femtosecond pulse over 610–668 nm is obtained. A 694 mW average output power of sum frequency generation is achieved for 4.1 W pump, representing 16.9% conversion efficiency at 615 nm.
From the point of view of the operation of motors,in order to eliminate the lower harmonics of PWM waveforms as much as possible,this paper develops the mathematical analysis model of PWM waveforms,determines the practical method of digital solution,and provides the program flowchart including the optimizing process.Finally,a set of calculating results is given.
Atmospheric pressure glow discharge was observed in a surface discharge generator. The frequency of ac power supply is more than 9 kHz and the sinusoidal peak-to-peak applied voltage is 9 kV. The electric field intensity in a kind of surface discharge generators is calculated with the boundary element method. Then a two-dimensional fluid model was used to simulate the ion trapping and electron trapping in a surface discharge just before the breakdown. The simulation results are in good agreement with our observation.
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