Instantaneous Beam Smoothing Technique Based on Beat Frequency of Broadband Laser

Objective In laser-driven inertial confinement fusion (ICF) facilities, the illumination uniformity of the laser beam is critical to the success of implosion. Varieties of beam smoothing schemes have been developed to improve illumination uniformity on the target. Typical temporal and spatial smoothing schemes, such as the combination of smoothing by spectral dispersion (SSD) and continuous phase plate (CPP), cannot suppress laser plasma instabilities (LPIs) on picosecond timescales. As rapid temporal smoothing schemes, our previous radial smoothing (RS) and azimuthal smoothing (AS) schemes can reduce the smoothing time to the picosecond level. However, RS has strict requirements on the pump laser, while AS demands precise wavelength differences between the sub-beams. Many studies have shown that broadband laser is helpful to break the wave vector matching in LPIs. To achieve the suppression of LPIs with broadband light and fast beam smoothing, we propose an instantaneous beam smoothing scheme based on the beat frequency of broadband laser. Methods Taking the National Ignition Facility (NIF) as a typical example, we build a physical model of the instantaneous beam smoothing scheme based on the beat frequency of broadband laser. In this scheme, four sub- beams successively propagate through a CPP array, a spiral phase plate (SPP) array, and a polarization control plate (PCP) array, and eventually focus on the target plane. It is noteworthy that four sub-beams are broadband laser. After passing through the SPP array and PCP array, four sub-beams are divided into two groups with orthogonal polarization. Meanwhile, every two sub-beams in a group have a conjugate spiral phase. Owing to the beat frequency of the rich frequency components of broadband laser and the conjugate spiral wavefront distributions, the speckles located at different positions on the target plane can rotate rapidly with different periods. In addition, the key factors of the proposed technique including bandwidth and spectrum type are analyzed in detail. Results and Discussions The results show that both the proposed scheme and the previous scheme based on the beat frequency of dual-frequency monochromatic laser have the same intensity envelopes of focal spots. However, the proposed scheme exhibits a faster smoothing rate (on a sub-picosecond timescale), fewer hot spots, and better illumination uniformity (Fig. 3). The reason for this phenomenon is the beat frequency of different frequency components of broadband laser. It means that the hot spots have different displacements with each other. In other words, the peak power of the hot spots decreases, and the illumination uniformity of the focal spots increases. In addition, with the increase in the bandwidth of the broadband laser, the smoothing rate can be significantly enhanced, while the illumination uniformity cannot be improved dramatically (Fig. 4). For instance, the focal spot is smoothed in a few picoseconds when Delta lambda = 0. 35 nm but in sub-picosecond at Delta lambda=3. 50 nm. However, both of them have close C-o. By means of shaping the spectrum type of broadband laser, the focal spot with Gaussian spectrum is smoothed more quickly than that with flat top spectrum (Fig. 5). Conclusions To meet the requirements of illumination uniformity in laser-driven ICF facilities, we propose an instantaneous beam smoothing scheme based on the beat frequency of broadband laser. In this technique with broadband laser as sub-beams, speckles at different positions on the focal plane rotate rapidly with different periods. The proposed technique can shorten the smoothing time to picosecond and even the sub-picosecond timescale by choosing appropriate parameters of the broadband laser. Compared with the technique based on the beat frequency of dual- frequency monochromatic laser, the proposed technique has more advantages attributed to the beat frequency of rich frequency components of broadband laser. The speckles on the target plane rotate more quickly, which leads to instantaneous smoothing of the focal spot and better illumination uniformity. On this basis, the effects of the key factors including bandwidth and spectrum type are analyzed. The results show that the smoothing performance of the proposed technique can be further improved by increasing the bandwidth of the laser and adopting the Gaussian spectrum of the broadband laser.