Study On Properties Of Reflection Volume Grating And Its Application In Spectral Beam Combining

With the development of laser technology and improvement of volume grating fabrication technique, volume gratings have a diverse range of applications such as optical information processing, optical communication system components and optical storge et al. In order to speed the applications of volume grating in various fields, it needs to thorough study the property of the volume grating from various aspects. In this thesis, on the one hand, the diffraction properties of reflection volume grating read by an ultrashort pulse beam are studied. On the other hand, the spectral beam combining of reflection volume grating are analyzed. The main research works are as follows:(1) Based on the modified Kogelnik diffraction efficiency equation, we study the diffraction intensity spectrum and the total diffraction efficiency of reflection volume holographic gratings (RVHG) in dispersion media. Taking photorefractive LiNbO3 crystal as an example, the effects of the grating parameters and the pulse width on the diffraction property are numerically analyzed with and without crystal material dispersion. The results show when pulse width is smaller or the grating spacing and the grating thickness are larger, the influence of dispersion on diffraction intensity and total diffraction efficiency are more obviously. Moreover, the diffraction intensity will decrease with dispersion effect.(2) The time-domain diffraction and transmission of RVHG recorded in photorefractive LiNbO3 crystals are theoretically studied and compared with the transmission volume holographic grating (TVHG) when they are illuminated by ultrashort pulse laser. The modified Kogelnik's coupled combining the dispersion effect in the crystal is used. The results show larger diffraction intensity can be obtained by choosing larger grating period, grating thickness and modulation index for a given pulse duration. The RVHG has high diffraction intensity than that of the TVHG as the same grating parameters. Also it is found that the diffracted pulse of the RVHG and the TVHG shift an opposite direction along the time-axis. The values of the shifts are functions of the readout pulse duration and grating parameters. The overmodulation phenomenon appears in the diffracted pulse of the TVHG at high values of the grating thickness and refractive index modulation, it doesn't occur in the RVHG.(3) The diffraction properties of RVHG are studied when the gratings are illuminated by an ultrashort beam with arbitrary temporal pofiles based on modified coupled wave theory. The dependences of the grating band, diffraction bandwidth and the total diffraction efficiency of the reflection volume grating on the temporal profiles of the input ultrashort pulse are investigated. Study shows that the bandwidth of the RVHG varies with the grating parameters more slowly than the TVHG. For three different temporal profiles, numerical results about diffraction properties of the reflection volume holographic gratings are given and compared with the TVHG.(4) The diffraction properties of RVHG are studied when the gratings are illuminated by an ultrashort beam with arbitrary polarization state. The modified Kogelnik's theory of coupled waves combining the dispersion effect in the LiNbO3 crystal is used. The dependences of the diffracted intensity distribution and the total diffraction efficiency of the RVHG on the polarization angle of the input ultrashort pulse are investigated. For different polarization angle, numerical results about diffraction properties of the reflection volume holographic gratings are given and compared with the TVHG.(5) A detailed model of the diffraction of plane and Gaussian beams on reflection volume Bragg gratings (RVBG) in photo-thermo-refractive glass based on Kogelnik's theory of coupled waves is presented. The physical model and the efficiency of spectral beam combining (SBC) for two channels, three channels and multi-channels by RVBG are investigated. Two and three channels SBC are numerically analyzed when the input beam divergence is 0.06mrad. The results show the SBC efficiencies are 98.65% and 97.57% when the spectral width is 0.1 nm, and the efficencies are 92.16% and 88.98% when the spectral width is 0.3nm, respectively. Particularly, the cross-talk and material losses, being the main contributing loss factors to the overall system efficiency, should be carefully addressed in the SBC design.