Search

With the development of the high power laser facilities, laser resistance of pulse compression grating is getting increased attention. The metal dielectric mixed grating (MDMG) is widely studied, because it has wide diffraction bandwidth, low surface stress and potential high threshold. The effect of nodular defect size on damage of MDMG is studied by experiments and theoretical analysis in two typical cases. The initial damages appear in the district of nodular defect due to local electric field enhancement induced by nodular defect, which is different with the initial damage located in grating pillar of the perfect MDMG. Laser induced ionization theory is introduced to describe the damage process and give a quantitative analysis on the relationship of laser damage and nodular defect size. It is concluded that local electric field enhancement in nodular defect area would cause the electron density to reach the critical value and lead to the damage occurrence, and nodule damage critical diameters are 1400 nm and 1500 nm respectively in case 1 (seed under the grating pillar) and case 2 (seed under the grating groove) which will lead to nodule damage initiation.

The gold-coated photoresist grating (GCPG) compressor is a core module of the high-repetition-rate superintense ultrafast lasers. Understanding how pulse trains with different repetition rates lead to laser-induced damage of GCPG is critical for future high-power laser applications. In this paper, we present the results of a multi-pulse laser-induced damage threshold (LIDT) investigation of GCPGs at different high repetition rates for the first time. The upper and lower endurance limits of S-on-1 beam-normal LIDT at 50–1000 Hz (800 nm, 35 fs) are ∼0.40 J/cm2 and 0.01 J/cm2, respectively, and the multi-pulse LIDT at 100 kHz is ∼0.02 J/cm2 corresponding to the peak intensity ∼64 GW/cm2 (1030 nm, 320 fs). The typical damage morphologies at 50–1000 Hz that the gold coating strips from the photoresist grating mask differ from those initiated from the expansion of the mask at 100 kHz. Focused ion beam scanning electron microscopy measurements show that the corners and small adhesion strength of interfaces are structural weaknesses. Moreover, theoretical modeling indicates that the differences mainly arise from pyrolysis and low heat-conduction of the mask. Overall, these results are highly practical and lay the foundation for the safe construction of high-repetition-rate ultra-high-power lasers based on the GCPG compressor.

This paper presents a sub-wavelength moth-eye-like antireflective structure on lithium triborate (LiB3O5, LBO) surface that can effectively promote the transmittance and maintain high laser-induced damage threshold (LIDT). Fabricated by holographic lithography and reactive ion-beam etching techniques, sub-wavelength periodic arrays can be obtained on the crystal surface. The surface morphology and transmittance were measured with atomic force microscope and spectrophotometer respectively. Single-sided transmittance of the LBO with moth-eye-like structure was confirmed to exceed 98% in the wavelength range of 1028–1300 nm, while is 98.73% at 1064 nm. For 1064 nm laser with the pulse width of 12 ns, the measured 1-on-1 LIDT is 42.57 J/cm2, which is about 2.28 times that of LBO compared with traditional antireflection (AR) coatings.

This work aims to design and fabricate sub-wavelength structures on lithium triborate surface for increasing its transmittance at 1064 nm. The finite-difference time-domain (FDTD) method is used to discuss the effects of subwavelength structure parameters such as period, depth, duty cycle and shape factor on the reflectivity. The subwavelength surface microstructures are fabricated by interference lithography technique and reactive ion beam etching method. The transmittance of the lithium triborate with single-side sub-wavelength structures is measured by UVvisible- near-infrared spectrophotometer. The results show that the transmittance of the sample with single-side microstructures at 1064 nm is about 4% higher than that of the polished substrate.

In a previous study, the temperature-rise of a spectral beam combining grating was analyzed theoretically and experimentally. It was concluded that the temperature of a grating can be effectively reduced by increasing the substrate thickness or by using a substrate material with higher thermal conductivity. In this study, yttrium aluminum garnet (YAG) was used as the substrate material to fabricate a spectral beam combining grating. The temperature, distortion, and far-field beam quality of the YAG-substrate-grating were analyzed theoretically and experimentally. It is concluded that, compared with the traditional quartz-substrate-grating, not only can the YAG-substrate-MDG withstand higher power, but the beam quality of the diffraction laser is also better.

In this work, a chirped volume Bragg grating (CVBG) with over 40 nm high efficiency broadband spectra for pulse compression of near 100 fs was studied. Based on the fundamental matrix method, the effects of various structural parameters of CVBG on its diffraction characteristics were analyzed and then a design of broadband and high efficiency CVBG was proposed. Afterward, the monolithic CVBG was utilized to stretch and recompress a 100 fs pulse with a center wavelength of 1030 nm. The result shows this device has a high diffraction efficiency (84%) and a fine reciprocity. For the fabrication of large-size broadband CVBGs, double cylindrical wave holographic interference in photo-thermorefractive (PTR) glass was applied to achieve a wide range uniform and stable light field, which could greatly minimize unnecessary space chirp.

With increased power of spectral beam combination, surface heat distortion of multilayer dielectric gratings (MDGs) could occur. In this study, the damage morphology of MDGs was initially analyzed under a continuous-wave laser irradiation. Subsequently, the surface distortion and temperature rise of different MDGs were tested experimentally. The experimental results showed that the initial damage of MDGs was caused by the thermal stress. Further, the thermal stress of the multilayer dielectric films on the MDG surface was analyzed theoretically. The calculated results were in good agreement with the experimental results. The conclusions indicated that with the increase of the MDG surface temperature, the stress in the HfO2 layers initially reached the stress damage threshold of the dielectric films and, therefore, the damage occurred.

This report is focused on laser induced damages and mechanisms of different pulse compression gratings(PCG) including multilayer dielectric gratings, metal gratings and metal-multilayer dielectric gratings. And the potential of metal-multilayer dielectric gratings is analysed.

A new 800nm center-wavelength metal dielectric grating with all SiO2 grating structures is designed and tested, which has a high laser damage threshold of 0.40J/cm2 on grating surface at pulse duration of 32fs.

In a spectral beam combination system, temperature increase of the multilayer dielectric grating (MDG) worsens the far-field beam quality of the output laser. To accurately monitor the surface temperature of the MDG, this study deposits V O 2 phase-change film on the lowest layer of multilayer dielectric films in the MDG and tests the transmittance with a probe laser. Based on this measurement, the surface temperature of the MDG can be calculated. Additionally, the study analyzes the influence of V O 2 film on the surface electric field and the − 1 diffraction efficiency of the MDG and presents a specific example of using V O 2 film to test high reflector temperature. The study concludes that V O 2 film is a feasible method of measuring temperature and better than an infrared thermal imager.

Spectral beam combination is a promising method for high-radiance lasers with a good beam quality. With the increase of the combination power, the temperature of the multilayer dielectric grating (MDG) unavoidably increases, leading to surface heat distortion of the MDG. In this study, the temperature field equation of the MDG is derived, and the key factors influencing the MDG temperature are investigated. Furthermore, experiments are performed to confirm the calculation results. The results reveal that the increase of the thickness of the substrate can improve the power tolerance of the MDG but delays the stable output of beam laser; use of a substrate material with a large thermal conductivity can greatly reduce the temperature of the MDG.

In this study, the realization of multiplexed volume Bragg gratings (VBGs) working as angle amplifiers in high power beam scanning system is theoretically and experimentally investigated. The design of the multiplexed VBG for the working wavelength of 1064 nm is described. We propose a cascaded multiplexed VBGs scheme that consists of 12 grating channels. Three 4-channel multiplexed VBGs were fabricated inside photo-thermo-refractive (PTR) glasses by multiple exposures and subsequent heat treatment. The test results show that this angle amplifier can achieve discrete angle deflection ranging from -45° to + 45°. The relative diffraction efficiency of all the grating channels is more than 80% and is almost polarization independent.

In previous research, the thermal distortion and far-field beam quality of a spectral beam combining grating were analyzed by theory and experiment under the irradiation of a high-power continuous-wave laser. It was concluded that the thermal expansion of the substrate was the main cause of the grating distortion and decrease in the beam quality. However, there was no further study to determine a method to decrease the heat deposition on the grating surface and far-field beam quality factor, M2. In this paper, we theoretically simulate the influence of the substrate thickness on the temperature field distribution and far-field beam quality of a multilayer dielectric grating. An experimental setup is proposed to verify the theoretical calculations. The experimental results are in good agreement with the calculations. The conclusions indicate that the temperature rise of the grating and M2 are effectively reduced by increasing the thickness of the substrate.

In this paper, we present a method for achieving precise evaluation of amplitude of refractive index modulation (RIM) inside the volume Bragg grating (VBG) recorded in photo-thermo-refractive (PTR) glasses. The Gaussian divergence characteristics of the incident beam is theoretically considered when calculating the angular selectivity of VBG, and the profiles of experimental angular selectivity curves are utilized to determine the value of RIM with one step. The effectiveness of our proposed method is experimentally verified. This method is applicable even if the full width at half maximum (FWHM) of VBG's angular selectivity curve has the same order of magnitude as or is less than the beam divergence.

Rational planning and improvement on the operational efficiency of the logistics park depends on the precise of definition of the radiation scope of the logistics park, but there is still a lack of scientific and accurate theoretical method aiming at solving the definition of the radiation scope of the logistics park. Based on the traditional space division model such as potential model in the economic geography as well as giving full consideration to the important role the transport network playing in the logistics activities, the accessibility was introduced into the traditional potential model based on the Euclidean distance metric to modify the distance parameter, and the method of defining the radiation scope of logistics park based on applying the accessibility analysis to potential model was proposed. Finally, on the basis of GIS software, this method was used to define the radiation scope of the five logistics parks of the similar type in Sichuan province. It proved that the application of this method based on GIS software had the advantages of the scientific accuracy as well as the easy acquisition and operation of data, which reflected the higher application value.

An opcode behavior based method is proposed to detect malware. Opcode behaviors are represented as opcode sequences from a decompiled executable. To accurately describe the malware behaviors, we construct the opcode running tree to simulate the dynamic execution of a program, and opcode n-grams are extracted to represent the features of an executable. The experimental results show that the opcode behaviors extracted by this method can fully represent the behavior characteristics of an executable. Compared with the detection method based the opcode distributions, the proposed method has higher overall accuracy and a lower false positive rate.

Transmission lines, microstrip and coplanar waveguide, have been fabricated on lossy silicon substrates embedded in CMOS Cu/SiO2 interconnect layers. The characteristic impedance, the loss and the slowing factor have been studied in detail with the geometric size of transmission lines. It is shown that microstrips and coplanar waveguides embedded in silicon oxide can be achieved with CMOS compatible interconnect technology and with low loss on lossy silicon wafers, which provide the essential passive components for designing microwave and millimetre-wave circuits on silicon wafers.

Thick copper (Cu)/Black Diamond™ (BD) layer up to 4 μm has successfully been integrated in CMOS interconnect process to improve the quality of on-chip RF passive components. It is shown that BD film is easy to crack when its thickness is up to 4 μm. However, by inserting one or few layers of dielectric material, BloK™, the stress in the entire dielectric film stack can be reduced. Although the reduction of the tensile stress of the stack is insignificant, the inserted BloK™ layer effectively prevents cracking from happening in the film stack. Spiral inductors have been integrated in developed Cu/BD (4 μm) top-metal-layer. Both Q value and resonate frequency of developed inductors are improved comparing to the inductors fabricated in previous top-metal-layer with 1 μm Cu/SiO 2 stack.