Study On Technologies Of Spectral Beam Combining And Low-pass Spatial Filtering

High-power laser with great beam quality has been significantly used in laserpumping, laser cutting and drilling, Inertial Confinement Fusion (ICF) system and so on.Spectral beam combining technology is an effective approach for achieving high powerlaser output, and the beam quality of the high power laser can be controlled or refinedby low-pass spatial filtering technologies. In this dissertation, the spectral beamcombining technology and low-pass spatial filtering technology for achieving highpower laser with great beam quality are studied, which provides a reference andrecommendations for achieving high power laser with great beam quality.The ultimate output power of single laser is limited due to thermal effects. It is aneffective approach for achieving high power laser output by spectral beam combiningtechnology, which incoherently combines several lasers with different wavelength into asingle near-diffraction-limited beam with the same aperture by using dispersive opticalelements and does not require phase control of each single laser. The spectral beamcombining approaches based on the transmitting volume Bragg grating (TVBG) andreflecting volume Bragg grating (RVBG) are studied and compared theoretically. Theresults show that in the spectral beam combining system with plane wave incidence, thesidelobes of the TVBG and the RVBG are the main factor which limits the spectralseparation between each laser channel and the spectral combining efficiency. In thespectral beam combining system with polychromatic beam incidence, the competitionbetween the spectral width of incident beam and the spectral selectivity of the TVBGand RVBG limits the spectral combining efficiency, and the sidelobes of the TVBG andRVBG are still the main factor which affects the spectral separation. For the TVBG,when the spectral width of the incident polychromatic beam is equal to40%times ofspectral selectivity of the TVBG, the diffraction efficiency is90%when the diffractionefficiency of the center Bragg wavelength is100%. For the RVBG, when the spectralwidth of incident polychromatic beam is equal to50%times of spectral selectivity ofthe RVBG, the diffraction efficiency is99.9%when the diffraction efficiency of thecenter Bragg wavelength is99.99%. Comparing with RVBG, there is additionalfar-field divergence for TVBG with polychromatic beam incident due to the dispersivecharacteristic of the grating, which limits the applications of TVBG in spectral beamcombining system.Several recommendations are proposed for solving the limitations of the spectralbeam combining technology based on the RVBG. In order to reduce the scale of thespectral beam combining system when the number of combined laser beam is huge, thefeasibility of spectral beam combining technology based on the superimposed reflectingvolume Bragg grating is demonstrated theoretically. For achieving the spectral separation near the pm levels, the spectral beam combining model based on thephase-shifted reflecting volume Bragg grating is established. For suppressing thesidelobes of the RVBG in spectral beam combining system, the spectral beamcombining model based on the apodisation reflecting volume Bragg grating is proposed.The thermal effects of the volume Bragg grating are considered. The results show thatthe scale of the spectral beam combining system can be effectively reduced byreasonable design the superimposed reflecting volume Bragg grating. By using thephase-shifted reflecting volume Bragg grating, the spectral separation near pm levelscan be achieved with the cost of incident angular controlling. The sidelobes of thereflecting volume Bragg grating can be effectively suppressed by using apodisationtechnologies. The peak diffraction efficiency of the first side lobe is suppressed from45%to0.8%after using the Sin apodisation approach when the diffraction efficiency ofthe no-apodisation RVBG is99.9%. The spectral separation between the two lasersources is also reduced effectively. For the RVBG with spectral selectivity is0.5nm,and the spectral width of the incident polychromatic beam is0.2nm, in order to achievethe spectral combining efficiency at95%, the spectral separation of the RVBG is0.94nm. However, the spectral separation is reduced to0.61nm after using the apodisationtechnology in RVBG. Using the apodisation technology, it is an effective approach toachieving high power laser output by increasing the number of combined beams in atotal spectral bandwidth. Using the angular diffraction characteristic and high laserinduced damage threshold performance of the reflecting volume Bragg grating, radiallypolarization beam is achieved theoretically with high combining efficiency bycombining two perpendicular linearly polarized beams with RVBG and phasecontrolling technology. The characteristic of the volume Bragg grating can be affectedby its temperature, and the temperature of the volume Bragg grating should becontrolled in spectral beam combining system.Low-pass spatial filter technology is an effective approach to refine or control thebeam quality of the laser beam. In high power laser system such as ICF, the pinholespatial filter limits the ultimate output power of the laser system with long pulse widthbecause of plasma closure of the pinholes. In order to control or refine the beam qualityof the high power laser system, the theory of low-pass spatial filtering based on thecylinder lens is established, and compared with the theory of pinhole spatial filter. Theresults show that the peak intensity of the incident beam in the focal plane of thecylinder lens decreases with increasing of the cylinder group distance. The peak powerin the focal plane of the cylinder lens is effectively reduced comparing with that of thelens with the same focal length in pinhole spatial filter. The peak power in the focalplane of the cylinder is smaller than that of the lens with same focal length in pinholespatial filter, and the pinhole closure can be effectively avoided in the same high powerlaser system. The filtering of high power laser with large beam aperture can be achieved by using holographic cylinder lens because it is convenient to achieve large scaleholographic cylinder lens. The results of filtering the deformed beam are completelysame by using the cylinder lens or lens with the same width of gaps and pinhole whenthe modulation frequency of deformed beam is larger than the cutoff frequency of thegap in low-spatial filter based on the cylinder lens, and the performance of pinholespatial filter in better than cylinder lens spatial filter when the modulation of thedeformed beam is larger than the cutoff frequency of the pinhole but smaller than that ofthe gaps. Numerical and experiment results show that the low-pass spatial filter can beachieved and the beam quality can be controlled or refined by using two groups ofcylinder lens with perpendicular focal line.In order to control or refine the beam quality of the high power laser which wouldcause plasma pinhole closure of the cylinder lens low-pass spatial filter system, a newfilter model by combining the RVBG and cylinder lens is proposed. In this new filtersystem, by using the RVBG to pre-filter the deformed beam, the intensity of the highangular frequency in the gap which located at the focal plane of the cylinder lens isreduced to avoid plasma pinhole closure. This new configuration provides a referenceand recommendations for low-pass spatial filtering technology in high power lasersystem.