Fabrication and analysis of broadband chirped volume Bragg grating

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.