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 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.
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