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Planetary gearboxes are widely used in the modern mechanical transmission equipment. The fault diagnosis of the planetary gearboxes is an important work. When the planetary gearbox fault occurs, its vibration signal has the characteristics of complex frequency components and amplitude modulation-frequency modulation. In view of these problems, a signal demodulation analysis method based on complementary ensemble empirical mode decomposition (CEEMD) and Teager energy operator is proposed to extract instantaneous amplitude and instantaneous frequency information of the fault signals. First, the CEEMD method is used to decompose the vibration signals of the complex multi component planetary gearbox into a series of single component intrinsic mode functions (IMFs). Then, the Teager energy operator is used to estimate the amplitude envelope and the instantaneous frequency of each single component signal. The intrinsic mode function is selected as the sensitive component according to the fluctuation characteristic of the instantaneous frequency. Finally, the Fourier transformation is applied in the obtained amplitude envelope and instantaneous frequency of the sensitive component to identify the gear fault characteristic frequencies, and achieve the accurate diagnosis of the planetary gearbox fault. The effectiveness and superiority of the method is verified through the analysis of the fault simulation experiment data of the planetary gearbox.

Frequency-domain optical parametric amplification (FOPA) is a new scheme that enables extremely broadband amplification of ultraintense pulses. The spatiotemporal coupling property of signal pulses can make the coherent noise of FOPA sharply different from that of conventional OPCPA. This paper presents a first theoretical study on the coherent noise produced in a FOPA system. We reveal that the coherent noise acquires the spatiotemporal coupling, and thus distinguishes the compressed signal pulse not only in time but also in space, which allows the suppression of coherent noise via optical manipulations in the spatial domain. The quantitative impacts of spatiotemporal coherent noise originated from the imperfections in either pump laser or crystal surfaces, are numerically studied. The result provides a new perspective on improving the coherent contrast of ultraintense lasers.

Gearbox is an important component of many industrial applications. When the gear fault occurs, the vibration signal is characterized by multi-component, multi-frequency modulation, low signal to noise ratio, weak fault characteristics and difficult to extract. This paper proposes a gear fault feature extraction method based on improved variational mode decomposition(VMD) and singular value difference spectrum. Firstly, the method is optimized for the decomposition level K of the VMD algorithm, and an improved method of VMD decomposition layer number K for central frequency screening (KVMD) is proposed. Then, the gear fault vibration signal is decomposed into a series of bandlimited intrinsic mode functions using KVMD. Due to the interference of the noise, it is difficult to make the correct judgment of fault in the spectrum of each mode component. According to the correlation coefficient criterion, the components with larger correlation coefficients are chosen to singular value decomposition. The singular value difference spectrum is obtained, and the effective order of the reconstructed signal is determined from the difference spectrum to denoise the signal; Finally, the processed signal is analyzed by Hilbert envelope. The fault characteristic frequency can be extracted accurately from the envelope spectrum. Through the analysis of the experimental data of gear fault, the results show that the method can effectively reduce the influence of the noise, and accurately realize the extraction of gear fault feature information.

The pulse contrast of state-of-the-art petawatt lasers is limited by coherent noise. This paper reports on a new family of noise, termed nonlinear beat noise, which is generated by the nonlinear mixing of two kinds of coherent noise in optical parametric chirped-pulse amplification (OPCPA). We theoretically study the various nonlinear beat noises and reveal their intensity evolutions in an OPCPA amplifier. The results suggest that nonlinear beat noise will be destructive to the future hundred-petawatt lasers.

The traditional method of kurtosis is calculating the kurtosis value in the time domain. The larger the obtained value is, the stronger the corresponding impact characteristic. Afterward, in order to characterize the period of the signal, the correlation kurtosis is proposed by combining the correlation coefficient and kurtosis. In this paper, the correlation kurtosis is calculated in the frequency domain to select the optimal analysis frequency band. But it has a poor performance in the case of low signal-to-noise ratio. Therefore, the maximum correlation kurtosis de-convolution method is applied as a preprocessing method to enhance impact characteristic. The whole band is divided into multiple sub-bands based on the wavelet packet binary tree and the optimal band is corresponding to the subband for which correlation kurtosis value is max.

Optical parametric phase (OPP), inherently induced in optical parametric chirped-pulse amplification (OPCPA), will significantly degrade pulse compression in the few-cycle pulse regime. While OPP is a well-known nonlinear phenomenon, its nonlinearity origin has remained unclear in physics. Here, we present a systematic theoretical investigation on OPP and demonstrate that OPP originates from the cascaded quadratic nonlinearity in a broadband OPCPA with intrinsic phase-mismatch. Because the intrinsic phase-mismatch is frequency dependent, OPP manifests itself as a nonlinear spectral phase. We show that OPP increases with the parametric gain, approaches to half the intrinsic phase-mismatch in the high-gain limit, and thus directly links with the crystal dispersion. We also find that the effect of OPP in the spatial domain is typically negligible, which makes the OPP compensation necessary only in the spectral domain. The results presented in this paper are of importance to guide the design of few-cycle intense OPCPA systems.

Optical parametric chirped-pulse amplification (OPCPA) has been demonstrated to be a promising approach for pushing femtosecond pulses towards ultra-high peak powers. However, the future success of OPCPA strongly relies on the ability to manipulate its phase-matching (PM) configuration. When a high average power pump laser is involved, the thermal effects in nonlinear crystals induce phase-mismatch distortions that pose an inherent limitation on the conversion efficiency. Here, we demonstrate that the noncollinear configuration previously adopted for wavelength-insensitive PM can be employed for temperature-insensitive PM when the noncollinear angle is properly reset. Simultaneous temperature- and wavelength-insensitive PM is realized for the first time by imposing such a temperature-insensitive noncollinear configuration with an angularly dispersed seed signal. Based on the lithium triborate crystal, the proposed noncollinear achromatic PM has a thermal acceptance 6 times larger than that of the conventional wavelength-insensitive noncollinear PM and has a sufficient spectral acceptance to support pulse durations of ~20 fs at 800 nm. These achievements open new possibilities for generating ultra-high peak power lasers with high average power.

A two-stage 2.2 μm OPCPA system is built with amplified chirped-pulse energy of 5 J. After compression, 1.8 J, 82 fs mid-IR pulses will be potentially obtained, providing an important driving source for strong-field physics. Article not available.

Second-harmonic generation (SHG) provides a promising route for generating vortex beams of both short wavelength and large topological charge. Here we theoretically investigate the efficiency optimization and beam characteristics of focused vortex-beam SHG. Owing to the increasing beam divergence, vortex beams have distinct features in SHG optimization compared with a Gaussian beam. We show that, under the noncritical phase-matching condition, the Boyd and Kleinman prediction of the optimal focusing parameter for Gaussian-beam SHG remains valid for vortex-beam SHG. However, under the critical phase-matching condition, which is sensitive to the beam divergence, the Boyd and Kleinman prediction is no longer valid. In contrast, the optimal focusing parameter for maximizing the SHG efficiency strongly depends on the vortex order. We also investigate the effects of focusing and phase-matching conditions on the second-harmonic beam characteristics.

In high-average-power second-harmonic generation (SHG) devices, the unavoidable thermal distortion induces a nonuniform phase mismatch that poses an inherent limitation on the conversion efficiency. Here we introduce a temperature-insensitive noncollinear (TIN) phase-matching scheme, which can significantly improve the performance of high-power SHG devices and is versatile to a broad range of laser wavelengths. In the proof-of-principle experiment with a lithium triborate crystal and a 1053 nm nanosecond laser, we demonstrate a large temperature bandwidth of ∼50 K×cm1/2 and a high SHG efficiency of ∼56%. This temperature bandwidth is 13 times that of the conventional collinear phase matching. We also numerically investigate the performance of the proposed TIN phase-matching scheme in high-power lasers. The demonstrated large temperature bandwidth allows efficient SHG in the high-power regime of 5–10 kW. The simplicity, high efficiency, and wavelength versatility will make the TIN phase matching attractive for wide applications in high-power lasers.

Due to the existence of spatial walk-off and/or group-velocity mismatch effects, pump-to-signal phase transfer becomes inevitable during parametric amplification. We experimentally demonstrate that in hybrid seeded optical parametric amplifiers (OPAs) that include two OPA stages seeded by the signal and idler waves, respectively, the phase of the output signal can be restored to its initial value, although there are spatial and temporal phase fluctuations on the pump source. This method significantly relaxes the requirement for high pump beam quality, which is always very stringent in parametric amplification systems. With the introduction of this scheme into birefringent phase-matching OPAs or chirped-pulse OPAs, it should be promising to achieve intense femtosecond laser pulses that are close to the diffraction limit in space and ultra-high contrast in time, simultaneously.

We experimentally study a new kind of parametric noise that is initiated from signal scattering and enhanced through optical parametric amplification. Such scattering noise behaves similarly to the parametric super-fluorescence in the spatial domain, yet is typically much stronger. In the time domain, it inherits the chirp of signal pulses and can be well compressed. We demonstrate that this scattering-initiated parametric noise has little influence on the amplified pulse contrast but can degrade the conversion efficiency substantially., 8 pages, 5 figures