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Interference noise affects the ranging capability of terahertz radar signal detection. Aiming at the above problem, a method based on adaptive empirical mode decomposition and stochastic resonance ...

A high-gain fiber amplifier at 1053 nm with a double-pass configuration is proposed. Compared to the Faraday-based double-pass structures, the pump efficiency is almost doubled, and the optical signal-to-noise ratio is improved as well. The effects of fiber length and the insertion loss of elements between two passes on the efficiency are studied experimentally, which is meaningful for the optimization of double-pass amplifiers. Using the fibered polarized beam splitters allow for the orthogonal propagation of forward and the backward signals on the same polarization-maintaining fiber, the polarization mode dispersion in the amplifier is hence totally avoided. With this structure, over 50-dB gain is obtained at pump power of 800 mW for a 1053-nm, 1-Hz nanosecond pulse.

Broadband low-coherence light has been applied in many fields such as optical imaging, atmospheric optical communication and laser inertial confinement fusion(ICF). It’s meaningful to investigate the temporal shaping technology of broadband low-coherence light. We propose and experimentally demonstrate a scheme of achieving all-optical arbitrary temporal shaping of broadband low-coherence light based on saturable absorption effect. Compared to traditional temporal shaping schemes such as electro-optic modulation(EOM) shaping or acousto-optic modulation(AOM) shaping, the modulation on the shaped pulse profile is much smaller, reduced from 14% (by acousto-optic modulator) to 3%(close to the ASE noise). Furthermore, we explore the evolution of shaped pulse contrast (intensity ratio between front edge and end edge) with the pump energy and contrast changing. The contrast of shaped pulse can be adjusted from 1:1.32 to 1:1.02 when pump contrast is 1:2, and from 1:2.25 to 1:1.90 when pump contrast is 1:3. And the maximum contrast of shaped pulse can reach 1:17 within the measurement accuracy. By changing the energy and contrast of pump, the contrast of the shaped pulse can be controlled flexibly. It’s useful to achieve all-optical arbitrary temporal shaping and pre-compensation during pulse amplification.

As the ultrafast laser has been developed, the measurement of ultra-speed transient and dynamic processes has gained much attention. An ultrafast oscilloscope based on the optical stretch method is promising to address this problem, but the measurement is restricted to the temporal profile. This means that the temporal phase is lost. In this work, we propose a full-field ultrafast oscilloscope using two temporal phase retrieval methods: the temporal annealing Gerchberg-Saxton (TAGS) algorithm and temporal ptychography. These could provide complete information, including temporal profile and phase, of high-rate repetitive transient pulses. The functions of an ultrafast oscilloscope with 230 GHz bandwidth and the two phase retrieval methods are verified by simulation and experimental results. This full-field ultrafast oscilloscope promises more applications in phase encoding, phase-contrast imaging, and sensing in the time domain.

Axle box bearings are important parts of rail vehicle running gear. Currently, there is limited research on the coupling modeling of an axle box bearing and rail vehicle. In this paper, a complete method for bearing modeling with multitype defects is proposed. The bearing kinematics with three translation and one rotation degrees of freedom are analyzed and modeled, and defect modeling algorithms for the inner raceway, outer raceway and rollers are proposed. The theoretical rolling tracks of rollers passing through the defect points are analyzed and formulated. By coupling the bearing model to the motion joint of the shaft end and axle box, a closed-loop bearing-vehicle model is established. Model simulations show that the calculated results for normal bearings, different types of bearing defects and the bearing-vehicle coupling model are consistent with the theoretical values. The novel and complete coupling model proposed in this paper has the advantages of having a simple algorithm, high computational efficiency, and accurate simulation of the bearing vibration response under train running conditions, which can provide important guidance for research on the response of an axle box bearing in train running gear. By establishing a bearing-vehicle coupling model that simulates the bearing vibration response, especially when the bearings have faults, the performance of the defective bearing can be better understood and applied to develop diagnostic algorithms for the condition monitoring of axle box bearings in a rail vehicle.

In this paper, we report on the influences of gain non-uniformity, Lyot filter effect and transmission spectrum of the passive fiber components on the spectral profile of a semiconductor saturable absorber mirror (SESAM)-based ytterbium (Yb)-doped mode-locked fiber laser. Simulation and experimental results prove that the adjustment of the Lyot filter will be an effective way to control the output spectrum during the actual application and manufacture of the mode-lock fiber laser.

Based on a transmittance model, we discovered that, by carefully changing the connection angle between the polarization-maintaining fibers in a laser cavity, the filtering effect effectively suppresses the Kelly sidebands caused by the strong nonlinear phase shift in a fiber mode-locked laser, thereby improving the signal-to-noise ratio of the mode-locked fiber laser. Compared with other spectral filtering approaches, the Lyot filter scheme, which has small insertion loss, simple structure, and adjustable modulation depth and transmission peak, can suppress the Kelly sidebands more effectively without affecting the shape of the mode-locked soliton. This was also verified experimentally. When the connection angle increases from 0° to 6°, the SNR of output pulses increases by 8 dB.

A high-energy, high-beam-quality, and pulse-width-tunable Nd:YAG laser system, pumped by vertical cavity surface emitting laser arrays and laser diodes, is demonstrated and applied to a velocity interferometry system for any reflector (VISAR) application in a high power laser facility. A multistage multipass amplification structure is used to fully extract the amplifier energy and obtain a high-energy pulse. The temporal waveform is compensated to provide a square waveform, with a flatness less than 8% (peak-to-peak value). The peak power is greater than 100 kW with a frequency-doubling efficiency of 25% for a 50 ns pulse width. The laser operates as a single shot with 1–5 Hz repetition frequency and 0.7% rms energy stability.

A high power laser system was used to drive the ignition of inertial confinement fusion (ICF), of which the high energy, the uniform focal spot, the accurate laser waveform, and the synchronization between the laser beams are key parameters. To accomplish this, global laser characteristics control should be assured, which was the main purpose of the injection laser system. In this paper, the key technological progress involved in the improvement of the performance of the injection laser of SG-II is reported, including frequency domain control, time domain control, near-field spatial shaping, pre-amplifier technology, and the optical parametric chirped pulse amplification pump source.

FM-AM modulation of high-power lasers significantly affects laser performance. Therefore, precise measurement of the FM-AM modulation depth is necessary. The subsequent FM-AM modulation generated by group velocity dispersion when the laser pulse propagates through a fiber affects the measurement accuracy. In order to eliminate this effect, a waveform-acquisition module is proposed that converts a broad-spectrum pulse of 1053 nm to a narrow-spectrum pulse of 1550 nm, without affecting the waveform. In addition, a signal-processing algorithm based on the orthogonal matching pursuit method is implemented to remove the sampling noise from the waveform. In this way, the signal-to-noise ratio of the measurement can be readily improved. Both theoretical and experimental results indicate that the proposed FM-AM modulation detection system is effective and economical. It can measure the FM-AM modulation depth precisely, and therefore shows considerable promise for future applications in high-power lasers.

FM-to-AM modulation is an important effect in the front end of high-power lasers that influences the temporal profile. Various methods have been implemented in standard-fiber and polarization-maintaining (PM)-fiber front ends to suppress the FM-to-AM modulation. To analyze the modulation in the front end, a theoretical model is established and detailed simulations carried out that show that the polarizing (PZ) fiber, whose fast axis has a large loss, can successfully suppress the modulation. Moreover, the stability of the FM-to-AM modulation can be improved, which is important for the front end to obtain a stable output. To verify the model, a PZ fiber front end is constructed experimentally. The FM-to-AM modulation, without any compensation, is less than 4%, whereas that of the PM fiber front end with the same structure is nearly 20%. The stability of the FM-to-AM modulation depth is analyzed experimentally and the peak-to-peak and standard deviation (SD) are 2% and 0.38%, respectively, over 3 h. The experimental results agree with the simulation results and both prove that the PZ fiber front end can successfully suppress the FM-to-AM conversion. The PZ fiber front end is a promising alternative for improving the performance of the front end in high-power laser facilities.

A fiber-based，high precision long-term stable time synchronization system for multi-channel laser pulses is presented，using fiber pulse stacker combined with high-speed optical-electrical conversion and electronics processing technology. This scheme is used to synchronize two individual lasers including a mode-lock laser and a time shaping pulse laser system. The relative timing jitter between two laser pulses achieved with this system is 970 fs (rms) in five minutes and 3.5 ps (rms) in five hours. The synchronization system is low cost and can work at over several tens of MHz repetition rate.

Shenguang-II 5PW laser is designed to deliver a 150J 30fs laser pulse into the target chamber. In beginning of 2016, 1PW (30J/27fs) outputs have been achieved and are used for physical experiments.

The temporal contrast levels of laser pulses in the injection laser system of the Shenguang-II facility are accurately measured and analyzed in detail. These signal-to-noise ratio levels are determined by the extinction ratios of an acousto-optic modulator, an electro-optic intensity modulator, and a combination of a Pockels cell and thin film polarizers. In addition, a drifting direct current bias voltage of an electro-optic intensity modulator and phase modulation are also demonstrated to affect the signal-to-noise ratio of the injection laser system. With a precise control of these factors, the injection laser system can output a nanosecond laser pulse with a signal-to-noise ratio of 47.37 dB. Moreover, a nanosecond laser pulse signal-to-noise ratio measuring device with a dynamic range and accuracy of 50 and 1 dB, respectively, is developed to detect the signal-to-noise ratio of a nanosecond laser pulse, which is 47.06 dB within an accuracy range of 1 dB. The signal-to-noise ratio measuring device can detect the high contrast laser pulse for the seed source unit in the high power laser facility accurately and conveniently.

We report a high-energy and high-gain fiber regenerative amplifier for narrow-bandwidth nanosecond laser pulses that uses a Yb-doped photonic crystal fiber. The input pulse energy is 270 pJ for a 3.5 ns laser pulse with 0.3 nm (FWHM) bandwidth. At a pump laser power of 8.6 W at 974 nm, pulse energies up to 746 μJ with 1.2% (rms) energy stability are generated. To the best of our knowledge, this is the highest energy obtained in a fiber-based regenerative amplifier. A high-energy, nearly diffraction-limited, single-mode beam with a high gain of 64 dB shows promise for future application in the front ends of high-power laser facilities.

A hybrid fibre-bulk regenerative amplifier with maximum output energy of 600uJ at 1Hz for narrow-linewidth nanoseconds laser is demonstrated. Nearly diffraction limited beam is obtained. The total gain is more than 63dB.

In order to characterize ultrashort pulses in real time at 1 mu m wavelength, a temporal imaging structure based on the four-wave mixing effect in highly nonlinear fibers is implemented and analyzed both theoretically and experimentally. It is found that both time-frequency transfer and the temporal magnification process can be realized approximately in one structure. The pulse widths of the signal laser measured by the time-frequency transfer and the temporal magnification process are 3.2 ps and 3.1 ps, respectively, which are nearly the same and are in agreement with the result of the autocorrelator. The temporal magnification factor is 33, and the temporal resolution is 380 fs. The method based on the temporal magnification process is inherently real time and single shot, which makes it suitable for applications in the measurement of high-power ultrashort pulses. The four-wave mixing time lens promises future applications in the characterization of the single-shot high-power short laser. (C) 2017 Optical Society of America

We propose a simple iterative algorithm based on a temporally movable phase modulation process to retrieve the weak temporal phase of laser pulses. This unambiguous method can be used to achieve a high accuracy and to simultaneously measure the weak temporal phase and temporal profile of pulses, which are almost transform-limited. A detailed analysis shows that this iterative method has valuable potential applications in the characterization of pulses with weak temporal phase.

In high-power laser system, in order to extend the components' service life and reduce the operation costs, more attentions should be pay at the research for damages ablation at multi-layer optical components and other high load optical components. 240ps, 35ps, 6ps 1053nm laser pulses has been used to investigate damage ablation and damage resistant experiments at 0° high reflection films. By comparing the damage morphology and damage resistant threshold of the ablation pits at different pulses width, it was superior to use ultra-short pulse to repair multi-layers optical components. It was found that the shorter pulse width has been used, the higher the damage resistant threshold and the lower the laser modulation. Furthermore, the finite-difference time-domain method was used to simulate the electric-field intensification within the large size damage region of multilayer films.

The calibration theory and measurement of four-terminal-pair (4TP) standard capacitor (STD CAP) frequency characteristic are studied for frequencies up to 13MHz. And frequency characteristic of 4TP air-dielectric standard capacitor 1000pF, 100pF, 10pF, 1pF and measurement uncertainty is provided

A fiber-based precision synchronization triggering system using fiber pulse stacker combined with high-speed electronics processing technology is presented. The relative timing jitter between two laser pulses achieved with this system is 4.09 ps (rms) in 2 h. The impact of the optical pulse amplitude fluctuation on the timing jitter is effectively reduced by high-speed analog-digital conversion and the reliability of the synchronization measurement system is confirmed.

An all-fiber regenerative amplifier at 1053 nm is demonstrated. The input signal pulse energy is 75 pJ in a 3.5-ns pulse at a 1-Hz repetition rate. At a low level of input pump power of 110 mW, the saturated output energy is 120 nJ with fluctuation less than 2% root mean square (RMS) even the fluctuation of the input pulse be about 15% (RMS). And the signal-to-noise ratio (SNR) is 66 dB. Maximum output energy of 780 nJ with a total gain of more than 40 dB is obtained at pump power of 130 mW. Scaling to higher pulse energy is constrained by stimulated Raman scattering. OCIS codes: 060.2320, 140.3280. doi: 10.3788/COL201210.S20610. As one of the most important components of a highpower laser system, the front end system seeds the subsequent laser system with pulses that have high stability, high beam quality, proper bandwidth, and high signalto-noise ratio (SNR). The output quality of front end determines the output quality of the whole laser system to a large extent. So developing a high-gain and high output stability amplifier with superior beam quality is attractive and important. Fiber optical amplifiers based on rare-earth-doped fibers have been investigated for decades [1 3] because of their inherent compactness, superior beam quality, and stability. Optical pulses with high repetition rate are routinely amplified by one or more staged single-pass rareearth-doped fiber amplifiers. However, the front end of high-power laser system runs in the state of extremely low repetition rate. The low-duty-cycle pulses leave large population inversion in the gain fiber for long durations between pulses, which can lead to serious broadband amplified spontaneous emission (ASE). The ASE competed for the unsaturated gain restricts the gain and degrade the SNR [4] .