High Power Laser Drive Control Of The Key Issues By Stimulated Raman Scattering

The effective control for various nonlinear optical effects during the propagation and amplification of high power laser pulses has been one of the focuses of the high power laser driver investigation. The stimulated rotational Raman scattering (SRRS) effect during the propagation of high intensity laser pulse through long air paths sets a limitation to the deliverable irradiation and propagation length, while the transverse stimulated Raman scattering (TSRS) effect in large-aperture KDP crystal, in combination with transverse stimulated Brillouin scattering (TSBS) effect in large silica optics limits the maximum third-harmonic output. These effects in addition to the self-focusing effect (including whole-beam self-focusing and small-scale beam breakup), contritube to main limitations of high power laser output ability and are called "power limits".In the construction of immense high power laser driver for fusion ignition with output laser energy up to mega joule, the guarantee of high beam quality, frequency conversion efficiency and safe operation entails effective control for nonlinear propagation effects. Meanwhile, the exploration of new approach for breaking up the "power limits" and improving the output ability would enrich the theory of high power laser and lend indispensable support to the substantial development of high power laser driver.As to improve the output ability and simultaneouslycut down the cost, the fluence of mega joule laser driver has advanced to (12～15) J/cm2 1ωand (6～8) J/cm2 3ω, with the adjustment from moderate saturation to deep saturation of amplification (FL≧3Fs). Besides, the scale-up of the driver adds to the difficulties of controlling stimulated Raman scattering (SRS, including SRRS and TSRS) effect during the propagation of high power laser pulse. One one hand, the direction of hundreds of laser beams to irradiate the target symmetrically makes long-path propagation of 1ωhigh power laser pulse inevitable. On the other, in the final optics system, which integrates a number of critical functions into a single compact package:frequency conversion, focusing, color separation, diagnostic beam sampling, vacuum isolation and debris shielding, although "shortening the propagation length of intense 3ωlaser pulse" helps to suppress SRRS effect, it brings in the difficulty to control various cycling "pollutions" (including stray light, organic volatiles and so on) contaminating the 3ωoptics during high fluence shots, adding to the risk of 3ωoptics damage, and limiting the maximum output. Therefore, within the safe propagation length of intense 3ωlaser pulse, distancing the 3ωoptics appropriately would improve the operation condition of 3ωoptics and load ability of the driver. All the above analyses indicate that the traditional evaluation of SRS based on laser pulse intensity and path length product (Intensity-Length product, IL) is not applicable in the construction of mega joule high power laser driver. Further investigation of the mechanism and law of the generalization and growth of SRS is necessary, as a solid foundation for the effective control of SRS effect.The mechanism and law of the generalization and growth of SRRS effect in long air paths and TSRS effect in large-aperture KDP crystal are extensively investigated in this paper, which lend sound theoretical and experimental support for the quantitative control of SRS effect in mega joule high power laser driver. A new effective control approach based on "transversely motive beam" is proposed, which has the potential to break up the "power limits" in high power laser driver.The spotlights of this thesis are as follows:1,The physical model of SRRS process is improved. Numerical simulation program is built which can quantitatively calculate the spatial-temporal characteristics during the generalization and growth of SRRS. Both the physical model and numerical code have been verified by corresponding experiments. Thus, this paper provides a reliable analyzing tool and experimental evidences for the quantitative control of SRRS effect in the design of mega joule high power laser driver.A four-dimensional numerical model allowing the tracing of the propagation of 1ωor 3ωhigh power laser pulse with random distribution of spatial and temporal profile is built to investigate the spatial-and-temporal evolvement of the laser pulse and the Stokes produced under the interaction of linear diffraction and nonlinear SRRS effect. The numerical model passed a number of reality checks based on related experimental results in and aboard as well as the recent experiment carried on SG-III TIL facility. The results show that all the spatial, temporal intensity modulation, aperture and pulse width will affect the growth of SRRS. Besides, the growth of SRRS has an obvious characteristic of threshold. Once～1% of the laser pulse energy was scattered into Stokes, the minor increase of propagation length leads to sharp energy loss and Stokes increase. In terms of temporal evolvement, the tail of laser pulse shifts to an earlier date and the pulse width is shortened. In terms of spatial evolvement, the near-field of Stokes shows a deep modulated speckle pattern and the peak intensity grows sharply up to several times of input intensity.2,Based on the mechanism and law of the generalization and growth of SRRS, a new control approach based on "transversely motive beam" is proposed and verified using the technology of "spectral angularly sweeping" as an example to generate "transversely motive beam". Some meaningful results convinced the scientific feasibility and showed the possibility to break up "power limits" in high power laser driver using "transversely motive beam".In the SRRS process developing from spontaneous emission noise, the initial Stokes light has a "noise" characteristic and those peaks across near filed of the Stokes are main contributors to the growth of SRRS. The concept of "transversely motive beam" is proposed to drive those peaks move transversely across the near field, therefore the SRRS effect located at these peaks will shift from steady-state response to transient range, leading to a sharp decrease of Raman gain. With the "transversely motive beam" generated by "spectral angularly sweeping"as an example, the feasibility of above concept is theoretically verified.3,Theoretical investigation of TSRS effect in large-aperture, high-fluence frequency conversion KDP crystal is carried out using improved physical and numerical model. The quantitative relationships between several key parameters and the growth of TSRS process are clarified, which provides an essential pre-condition to the investigation of effective control measurements of TSRS.A numerical model for the simulation of TSRS effect in large-aperture optics is built and verified. The Stokes distribution in the crystal calculated with this model has several similarities with the experimentally observed damage pattern of the crystal, which shows a link between TSRS effect and the damage to large-aperture crystals in high-fluence operation. The analytic relationship between the intensity of the Stocks light and, the pulse width, and aperture of pump laser as well as the edge reflectivity of the large-aperture crystal has been deduced, which facilitates the evaluation of TSRS effect and effective control of TSRS in the high power laser driver.The numerical model for both SRRS and TSRS effect built in this paper has passed a number of "reality checks", however, more code verifications is still needed for further improvement. As for the experimental investigation, the SRRS effect of high intensity pulse-shaped laser beam in long air path has received quantitative study, while the TSRS effect has only been observed in experiments with qualitative phenomenon. Besides, although the concept of controlling nonlinear propagation effect based on "transversely motive beam" is proposed and preliminarily verified using the example of "spectral angularly sweep" beam, it still demands further and extensive study both theoretically and experimentally. Other possible technology for the generation of "transversely motive beam", the physical mechanism and characteristics during the linear and nonlinear propagation of "transversely motive beam" and the interaction between linear diffraction and nonlinear effects as well as the approach for controlling the evolvement of the spatial-temporal profile during propagation all deserve further theoretical and experimental investigation.