tailieunhanh - Coherence and Ultrashort Pulse Laser Emission Part 9

Tham khảo tài liệu 'coherence and ultrashort pulse laser emission part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 312 Coherence and Ultrashort Pulse Laser Emission low reflectivity is used for the prepulses and the pedestal while a several orders of magnitude higher reflectivity value is applied for the main pulse. This fast plasma shutter is well suited for suppression of unwanted light before the main pulse. Consequently the contrast of the pulse is increased by the ratio of the plasma reflectivity to cold or Fresnel surface reflectivity of the material. The contrast improvement is typically 2 to 3 orders of magnitude with AR coated targets and s incident polarization or in a geometry with an incidence angle close to Brewster s angle and p-polarization. If the plasma scale length -see Eq. 2- exceeds the laser wavelength the plasma starts to absorb and distort the phasefront of the reflected pulse leading to lower reflectivity and the loss of beamed specular reflection Horlein et al. 2008 . The principle of the plasma mirror is illustrated in Fig. 4. The plasma mirror Kapteyn et al. 1991 is used to improve the laser pulse after amplification and compression and provides higher throughput without limitation on the input energy Gibbon 2007 . Since it is used after the whole laser system the plasma mirror can be implemented without any modification to the system itself. Further advantages are wide bandwidth acceptance as will be discussed later Nomura et al. 2007 and spatial filtering effect if the plasma mirror is in the vicinity of the laser focus Gold 1994 Doumy et al. 2004a Horlein et al. 2008 but no smoother beam profile or even degradation was reported using the target in the near-field Dromey et al. 2004 Horlein et al. 2008 . Several investigations in different geometries Backus et al. 1993 Ziener et al. 2002 Doumy et al. 2004a as normal 45 and Brewster s angle of incidence were conducted to study the reflectivity of the plasma mirror yielding 50-80 overall -time- and space-integrated- energy reflectivity and a measured contrast enhancement of 50-100 for s-polarization .