Recent Progress and Development Trend of Self-Sweeping Fiber Laser

Since 2011, when Kir’yanov et al. first reported a new wavelength self-sweeping ytterbium-doped fiber laser that does not rely on any tuning element but only on the dynamic induced grating generated in the gain fiber by the standing wave resonator structure, the self-sweeping effect based on fiber waveguides has been extensively studied, leading to great progress in fundamental physics and other applications of self-sweeping fiber lasers. Different doped fiber lasers have not only achieved the self-sweeping effect, but also observed new phenomena such as anomalous self-sweeping and continuous pulses. Due to their remarkable spectral and pulsed characteristics, self-sweeping fiber lasers have been widely used in spectral detection, fiber sensing and short pulse synthesis. In this paper, we will introduce the classification of different doped self-sweeping fiber lasers, summarize their different implementations, and introduce their self-sweeping laws, pulse characteristics, recent progress of applications and future development prospects.

Study on pump pulse characteristics of QCW diode-side-pumped Nd:YAG laser

The influence of pumping laser pulse on the property of quasi-continuous-wave diode-side-pumped Nd:YAG laser is investigated theoretically and experimentally here. Under remaining a fixed duty cycle, the average output power increases, and the corresponding thermal focal length shorten with the increase of the pump pulse duration, which attributes to the decrease of the ratio of pulse buildup time to the pulse duration. At a pump power of 146 W, the laser output power changes from 65.1 W to 81.2 W when the pulse duration is adjusted from 150 μs to 1000 μs, confirming a significant enhancement of 24.7%. A laser rate equation model incorporating the amplified spontaneous emission is also utilized and numerically solved, and the simulated results agree well with the experimental data.

Output Pulse Characteristics of a Mamyshev Fiber Oscillator

The dependence of the output pulse characteristics of a Mamyshev fiber oscillator on cavity parameters is investigated in detail. We analyze the change in pulse spectrum bandwidth, pulse duration, dechirped pulse duration and chirp with the change in fiber group velocity dispersion, fiber nonlinearity, gain, and filters by putting forward a numerical model. In particular, as one of the most important components, the effect of filters bandwidth and the central wavelength interval between them is discussed. The passive fibers are classified into two kinds according to their locations in the cavity, which are the one before the gain fiber and the one after the gain fiber. Numerical simulation results show that a wide spectrum can be obtained by increasing the nonlinearity of the second passive fiber, while the change in nonlinearity of the first passive fiber has a weak effect on spectrum broadening. A wide spectrum could also be obtained by increasing the nonlinearity or the small-signal gain coefficient of the gain fiber. A Yb-doped Mamyshev fiber oscillator is demonstrated. The results show the increase in pump power, which agrees reasonably well with the numerical simulation results.

Tunable Spacing Dual-Wavelength Q-Switched Fiber Laser Based on Tunable FBG Device

A tunable spacing dual-wavelength Q-switched fiber laser is experimentally demonstrated based on a fiber Bragg grating tunable device incorporated in an erbium-doped fiber laser (EDFL). The system utilizes two identical fiber Bragg gratings (FBGs) at 1547.1 nm origin to enable two laser lines operation. The wavelength separations between two laser lines are controlled by fixing one of the FBGs while applying mechanical stretch and compression to the other one, using a fiber Bragg grating tunable device. The seven steps of wavelength spacing could be tuned from 0.3344 to 0.0469 nm spacing. Pulse characteristics for both close and wide spacing of dual-wavelength Q-switched fiber laser are successfully being recorded. The findings demonstrate the latest idea of dual-wavelength fiber laser based on FBG tunable device, which offers a wide range of future applications.

Influence of low-energy proton irradiation on the effective lifetime in the space charge region of silicon n+-p junctions

The effect of low-energy proton irradiation on the pulse characteristics of silicon n+-p-p+ structures is analyzed. It is shown that irradiation with protons with an energy of 180 keV and a dose of 1015 cm−2 creates a region with an effective lifetime of 5.5·10−8 s in the space charge region of the n+-p junction. Such elements can be used to create high-speed photodiodes with an operating modulation frequency of 18 MHz.

A Generic Sequential Stimulation Adapter for Reducing Muscle Fatigue during Functional Electrical Stimulation

Background: Clinical applications of conventional functional electrical stimulation (FES) administered via a single electrode are limited by rapid onset neuromuscular fatigue. “Sequential” (SEQ) stimulation, involving the rotation of pulses between multiple active electrodes, has been shown to reduce fatigue compared to conventional FES. However, there has been limited adoption of SEQ in research and clinical settings. Methods: The SEQ adapter is a small, battery-powered device that transforms the output of any commercially available electrical stimulator into SEQ stimulation. We examined the output of the adaptor across a range of clinically relevant stimulation pulse parameters to verify the signal integrity preservation ability of the SEQ adapter. Pulse frequency, amplitude, and duration were varied across discrete states between 4 and 200 Hz, 10 and100 mA, and 50 and 2000 μs, respectively. Results: A total of 420 trials were conducted, with 80 stimulation pulses per trial. The SEQ adapter demonstrated excellent preservation of signal integrity, matching the pulse characteristics of the originating stimulator within 1% error. The SEQ adapter operates as expected at pulse frequencies up to 160 Hz, failing at a frequency of 200 Hz. Conclusion: The SEQ adapter represents an effective and low-cost solution to increase the utilization of SEQ in existing rehabilitation paradigms.

Influence of Pulse Characteristics On Turbocharger Radial Turbine

Due to the reciprocating engine, a pulsating flow occurs in the turbine turbocharger, which experiences conditions far from the continuous flow scenario. In this work, the effects of the characteristics of the mass flow pulse, parameterized through amplitude, frequency and temporal gradient, are decoupled and studied via unsteady Computational Fluid Dynamics calculations under on-engine operating conditions. Firstly, the model is validated based on comparisons with experimental data in steady flow conditions. Then, the effect of each parameter on exergy budget is assessed by considering a +/-10% variation with respect to a baseline pulse. The other factors defining the operating conditions (e.g. mass flow, shaft speed and inflow exergy) are kept the same as the baseline. The adopted approach enables to completely isolate the effects of each parameter in contrast with previous literature studies. Based on the results observed, pulse amplitude is identified as the primary parameter affecting the hot-side system response in terms of turbine performance, heat transfer and entropy generation, while frequency and temporal gradient show a smaller influence compared to it. As the pulse amplitude increases, the turbine work is reported to improve up to 9.4%. Smaller variations are observed for the frequency and temporal gradient analysis. With a 10% increase of the pulse frequency the turbine work is registered to improve by 5.0%, while the same percentage reduction of the temporal gradient leads to an increase of turbine work equal to 3.6%.