Reflective Volume Bragg Grating (RBG) recorded in photo-thermo-refractive glass offers notable advantages, including high efficiency, narrow bandwidth, and multiplexing capabilities, making them ideal for applications in dense wavelength division multiplexing (DWDM) and demultiplexing. However, the side lobes caused by sudden changes in coupling strength at both ends of the grating always lead to interchannel interference, which evidently affects the effectiveness of multiplexing and demultiplexing. In this work, a theoretical model of the apodization for RBG, which based on Kogelnik's coupled wave theory and F-matrix theory. It is simulated and analyzed the effects of three apodization functions including cosine, gaussian, and hyperbolic secant functions is established. The simulation results indicate that the side lobes of the RBG with apodization is significantly reduced and the peak efficiency is determined by the total refractive index modulation. This work provides a theoretical design basis and parameters optimizing method for the development of apodized RBG.
The pulse compressor has evolved to be a core module of 100 Petawatt (PW) or Exawatt laser facilities; however, the diffraction efficiency bandwidth, laser-induced damage threshold (LIDT), and aperture of its currently deployed gratings strongly restrict the ultra-intense and ultrashort pulse compression. Maximizing the energy-loading capability of gratings is a worldwide challenge in the high-peak-power laser field. Here, for 10’s femtosecond-PW pulse compression, an ultra-broadband gold grating was developed to dramatically broaden the high diffraction efficiency bandwidth from 100–200 nm to 400 nm. Moreover, a core mechanism was elucidated whereby the high diffraction efficiency combined with the deep penetration effect of electrons under high-energy laser irradiation can tap the LIDT potential of metal grating. Accordingly, the mixed metal grating was invented and demonstrated experimentally with superior performance in terms of diffraction efficiency bandwidth and LIDT compared with normal gold gratings. In addition, for picosecond-PW pulse compression, a multilayer dielectric grating (MDG) design paradigm was proposed. Importantly, TM-polarized MDGs had the superiority of a high LIDT owing to the low electric field intensity. Furthermore, a novel grating with a LIDT 3.5 times higher than the conventional gratings installed in NIF-ARC and SG-II was obtained by taking advantage of TM polarization and a small incident angle. These results make a pioneering technical reserve to facilitate future 100 PW-class ultrafast laser systems.
Meter-scale optical films have become core components in large-size optical systems. Improving film thickness uniformity is a crucial issue to ensure optical properties. Ag films are commonly used as high-reflection mirrors for optical systems. Here a meter-scale magnetron sputtering equipment has been independently developed for coating Ag films. Two strategies were performed to correct the film thickness uniformity in the horizontal direction and vertical directions respectively. Specifically, the film thickness uniformity at the horizontal position was guaranteed by scanning the target parallel to the horizontal direction. Meanwhile, a mask was designed to optimize vertical film thickness uniformity. The film thickness, fitted by X-ray reflectometry (XRR), non-uniformity is less than 3% both in the 1800 mm range of the horizontal direction and the 1100 mm range of the vertical direction. This study has brought new insight into the development of meter-scale thin film optics.
Compressing high-energy laser pulses to a single cycle will break the current limitation of super-scale projects and facilitate future Exawatt lasers. However, the lack of ultra-broadband gratings capable of supporting single-cycle pulse stretching and compression is a core problem that cannot be overcome. Recently, we solved this problem and developed gratings with the broadest bandwidth (broader than 400 nm) that can support near-single-cycle laser pulses (about 4 fs). A 200 mm (Length) ultra-broadband grating is being fabricated supporting a single-cycle terawatt-level prototype. Meanwhile, the 1600 mm (Length) fabrication campaign is underway and will be completed in 2024–2025 at SIOM, facilitating the single-cycle Exawatt lasers.
The volume Bragg grating (VBG) recorded in the photo-thermo-refractive (PTR) glass has high diffraction efficiency (DE), excellent angle selectivity, multiplexed and flexible design, which is an ideal device to achieve the angle magnification of beam scanning. In this study, a 4-channel multiplexed VBGs with the average relative diffraction efficiency (RDE) greater than 96% at 1064 nm and the maximum discrete angle deflection of 12° was designed and fabricated. And the angle deviation of experiments and design schemes were controlled less than 0.3°. The laser damage of PTR glass and multiplexed VBGs were also tested at 1064 nm. Laser damage tests were performed with a beam diameter of 0.3 mm by "1-on-1" mode. It showed that the laser damage threshold of PTR glass and multiplexed VBGs were 44.33J/cm2 and 30.15 J/cm2 respectively.
In this work, based on the double cylindrical wave holographic interference method, a broadband chirped volume Bragg grating (CVBG) in photo-thermo-refractive glass (PTR) has been fabricated and studied, which has a diffraction bandwidth of about 23.8 nm. The transmittance and diffraction efficiency (DE) of the prepared CVBG were measured. The results showed that the DE and refractive index modulation (RIM) of the CVBG samples increased first and then decreased with the increase of the heat treatment time. And the transmittance of all samples decreased, indicating that the losses (sum of absorption and scattering) of the samples increased. It was found that the absorption coefficients of all samples did not exceed 0.1cm-1 even after prolonged heating, and the scattering losses accounted for the main part losses of the CVBG. Then, the thermostatic duration of the sample was shorter during heat treatment at the first time, the starting point of the RIM of CVBG became higher, which is expected to obtain higher RIM and DE of the CVBG in the subsequent heat development process. Although increasing the dose of UV exposure can reduce the losses of the CVBG, it is not conducive to the improvement of the RIM and DE of the CVBG. Therefore, we have made restrictions on both the dose of UV exposure and the duration of heat treatment. Finally, based on the fundamental matrix (F-matrix) method, the influence of the key structural parameters of the CVBG on its diffraction characteristics was analyzed, and a guidance scheme for making high-efficiency CVBG was proposed.
Vortex volume grating (VVG) has advantages of high diffraction efficiency (DE), excellent angular selectivity, broad spectral band, and flexible design, making it a good candidate as an orbital angular momentum (OAM) generator in high power laser system.. In our research, a vortex volume grating with a relative diffraction efficiency of 84.58% and an angular spectrum of 1.864 mrad at 1064 nm is fabricated, and the refractive index modulation (RIM) is retrieved.
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.
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.
Diffraction efficiency and its uniformity are both the important performance indicators of a large-aperture pulse compression grating (PCG), and their accurate measurement is of great importance for evaluating the performance of a PCG and improving the manufacturing technique of a PCG. In this paper, based on the traditional double beam photometric measuring framework, and in combination with synchronously scanning in two-dimension the optical fiber and the detector (small size, light weight), fast measurement for the diffraction efficiency and its uniformity of a largeaperture PCG was realized. Experimental result shows that for a PCG in the size of 1400×430 mm, when the sampling intervals in both the horizontal and the vertical directions are 10 mm, the whole measurement takes about 3 hours, and a repeatability error of about 0.1% is obtained with the corresponding experiment. Compared with the currently popular measuring strategy by scanning the large-aperture PCG (large size, heavy weight), our proposed measurement system has the advantages of significantly increasing the data acquisition speed, and reducing the cost of system construction, meanwhile the safety loophole is greatly eliminated.
Spectral beam combining (SBC) laser systems are rapidly advancing the output power scaling for high-average-power beam-combined fiber lasers with near-perfect beam quality. Grating as the dispersive element requires high diffraction efficiency (DE) and broad bandwidth (BW) for improving the combined output power scaling. A broad BW high DE polarization-independent reflective multilayer dielectric (MLD) grating is designed. The -1st order DE of the designed grating exceeds 99.84% at 1053nm central wavelength. The BW of the grating with the -1st order DE higher than 99% is up to 99 nm for non-polarized incident light. This MLD grating can be used as a reliable combiner in the SBC system.
Owing to damage, thermal issues, and nonlinear optical effects, the output power of fiber laser has been proven to be limited. Beam combining techniques are the attractive solutions to achieve high-power high-brightness fiber laser output. The spectral beam combining (SBC) is a promising method to achieve high average power output without influencing the beam quality. A polarization independent spectral beam combining grating is one of the key elements in the SBC. In this paper the diffraction efficiency of the grating is investigated by rigorous coupled-wave analysis (RCWA). The theoretical -1st order diffraction efficiency of the grating is more than 95% from 1010nm to 1080nm for both TE and TM polarizations. The fabrication tolerance is analyzed. The polarization independent spectral beam combining grating with the period of 1.04μm has been fabricated by holographic lithography - ion beam etching, which are within the fabrication tolerance.
Polyvinyltoluene-based nanocomposite plastic scintillators containing uniformly dispersed ytterbium fluoride
nanoparticles are prepared by in-situ thermal polymerization. The deep-blue-emission nanocomposite monoliths
are capable of producing a full energy phtotpeak when exposed to Cs-137 (662 keV) gamma radiations. The effects
of monolith dimension and nanoparticles content on scintillation light yield and energy resolution are studied. A
light yield of 65% (compared with the commercial standard Eljen-212) and a photopeak energy resolution of 9.2%
was obtained using a sample loaded with 24.5 wt% ytterbium fluoride nanoparticles.
The multilayer ridge metal/multilayer-dielectric gratings (MMDGs) for pulse compressors show high efficiency, broad bandwidths, large fabrication tolerances and high laser-induced damage thresholds. The diffraction efficiency, bandwidth, and near-field distribution of the multilayer structure ridge MMDG are theoretically investigated. Simulation results show that the film structure of the grating ridge has a great influence on the bandwidth and near-field distribution. The maximum electric field is located in the high-index layer of the grating ridge with high -1st diffraction efficiency. As the thickness of the high-index layer decreases, the maximum electric field moves to the low-index layer of the grating ridge with. Base on the results, the sandwich ridge MMDG is an ideal pulse compression grating for chirped pulse amplification systems.
In this paper, metallic pulse compression gratings (MPCG) with three kinds of grating structures are manufactured. The diffraction efficiency and bandwidth of samples are measured and it can be found that the different grating structure has different diffraction efficiency and bandwidth. Laser damage tests of samples are implemented by an 800±30 nm laser at pulse duration of 31 fs and it can be also found that the different grating structure has different laser-induced damage threshold. Experimental measurements illustrate that the grating structure has a great influence on bandwidth, diffraction efficiency and damage threshold. The typical damage morphologies of MPCG reveal that the damage is induced by absorption and thermal stress.
Metal multilayer dielectric gratings (MMDGs) for pulse compressors used in high-energy laser systems should enable
high efficiency, as well as provide broad bandwidths and high laser-induced damage thresholds. The non-uniform optical
near-field distribution of MMDGs is an important factor that limits damage resistance capabilities. The efficiency and
electric field distributions of MMDGs with a corrugated SiO2 layer and operated at 800 nm are analyzed by rigorous
coupled-wave analysis. The maximum electric field in the grating ridge, match layer, and metal layer decreases with
increasing grating diffraction efficiency. High efficiency and a low electric field are obtained with a 90° slope angle in
the grating ridge. The bandwidth and maximum electric field in the metal layer decrease with increasing high- and
low-index material pairs, and the maximum electric fields in the grating ridge and match layer initially decrease and then
increase. The peak electric field in the grating is optimized with a merit function; the optimization covers the
enhancement of diffraction efficiency, bandwidth, and reduction of electric field. The bandwidth of the optimized
MMDGs is 160 nm with a diffraction efficiency exceeding 90%. The largest electric field is modulated in air to obtain a
low electric field and high laser-induced damage threshold.
Multilayer dielectric gratings (MDGs) are more and more used to compress pulse in the next generation
of chirped-pulse amplification (CPA) system for high-energy petawatt (HEPW)-class lasers due to their
high efficiency and high damage threshold for picosecond pulses. The damage tests for MDGs were
carried out with long pulse (12ns) in air and short pulse (0.66~9.7ps) in vacuum at 1053nm,
respectively. The experiment methodologies and results were discussed. For both long and short pulse,
the initial damage locates at the grating ridges opposite to the incoming wave, which is consistent with
the maximum normalized electric field intensity (NEFI). For long pulse, the damage is characterized by
melting and boiling. And for short pulse, the damage is ascribed to multiphoton-induced avalanche
ionization because of the electric field enhancement in the grating groove structure. And Measurement
results of the dependence of damage threshold on the pulse width are presented. And the damage
threshold of MDG in beam normal is 4.4J/cm2 at 70° incidence angle for 9.7ps pulse.
The multilayer dielectrics (MLDs) for broad bandwidth 800nm pulse compression gratings were
fabricated with optimized design by electron beam evaporation using three different kinds of materials
(Ta2O5/SiO2/HfO2), which had more than 99% reflectance with bandwidth larger than 160nm around
the center wavelength of 800 nm and high transmission at the exposure wavelength of 413nm.
Laser-induced damage behaviors of the mirrors were investigated. It was found that the laser-induced
damage threshold (LIDT) of the samples could reach 1.0J/cm2 and 2.0J/cm2 in the normal beam (57
degrees, TE mode) at pulse duration of 50fs and 120fs, respectively. The depth information of the
damage sites at these two cases was explored by atomic force microscope (AFM). The reason of the
sample having so high LIDT was also discussed in this paper. The MLDs provide a solid base for the
high laser threshold 800nm pulse compression gratings and may open a new way for broad bandwidth
800nm reflectance coatings used in the ultrashort pulse laser system.
In this paper, porous nanostructures on BK7 glass were manufactured by chemical treatment in order to obtain antireflection (AR) components with improved laser damage resistance. The damage-resistant properties of the samples with nearly 100% transmittances at three pulsed laser wavelength were investigated. The damage tests showed that the BK7 glass with AR nanostructures can achieve the LIDTs of 58J/cm2, 20 J/cm2 and 12 J/cm2 under the irradiation of 12ns 1064nm pulses, 10ns 532nm pulses and 8ns 355nm pulses, respectively. These values are much higher than those of AR coated glasses, but are almost the same level of un-etched substrate. The effect of structural properties on electric field distribution of porous surface was investigated by a three-dimensional (3D) finite difference time-domain (FDTD) model. The simulation results and the morphology of damage site on porous glass are compared to those of un-etched surface, and are discussed to reveal the possible damage mechanism. Finally, some possible solutions to improve the LIDT are proposed.
The internal electric field enhancement is critical for the laser induced damage properties of pulse compression gratings
(PCG) in high-energy laser systems. Due to complex fabrication processes of PCG such as coating, interference
lithography and etching, different kinds of defects, like nodular defects in multilayers and non-uniformities of the grating
profiles on PCG surface, can't be practically avoided. From simulation results, we can know that some of these defects
have little effect on the spectral response of optical elements, but they may produce huge changes of internal electric
fields and thus decrease the damage threshold of PCG. To obtain a better understanding of the dependence of the internal
electric field enhancement on these defects and their dimensions, this work is focused on the near field distributions of
defective PCGs using rigorous electric magnetic field (EMF) solvers.
HfO2 films have been deposited with electron beam evaporation of oxide hafnium, reactive evaporation (RE) and reactive ion beam assisted deposition (RIBAD). The optical and structural properties and laser-induced damage threshold of the films have been studied. It was found that HfO2 film deposited with RE has less defects and good uniformity. In addition, the samples deposited with RIBAD have higher refraction index, and can reduce the absorption at a certain condition. Laser-induced damage (LIDT) is under improvement. The crystal structure of the sample deposited with RIBAD is monoclinic, and when the bombardment energy raises, the preferred orientation changes from (002) to (-111).
Single layers of MgF2 were deposited upon super polished fused-silica substrates by resistance boat evaporation at different temperatures. The transmittance and reflectance spectra were measured and the optical losses were analyzed. The laser-induced damage thresholds (LIDT) were characterized at the wavelength of 355nm for certain samples. The absorbance of the samples was discussed related to the LIDT. The increasing absorbance with the higher deposited temperature was attributed to the decreasing of the LIDT. And the high micro-defect density of the sample surface resulted in the low LIDT.
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