Doppler-induced continuous spectral broadening of ultraviolet lasers

Abstract

We propose a compact scheme based on ultrafast-rotating phase plates (URPPs) to achieve continuous spectral broadening of ultraviolet lasers. The rapid rotation elements behave as a random oscillator which induces Doppler frequency shift into the ultraviolet lasers. As an example, for a disk-shaped phase plate, with the beam acting on the edge at a radius of 10 cm, a rotation frequency of 1 kHz, and a phase-element size of 10 nm, the continuous spectral broadening reaches 0.07%. Further increasing the rotation speed or reducing the phase-element can lead to greater spectral broadening. When multiple URPPs are arranged in series, the superimposed spatiotemporal modulation further enhances the continuous spectral broadening and achieves more effective speckle smoothing. The scheme is applicable to broadening the independent spectrum of optical frequency combs as well as to the mitigation of laser-plasma instabilities in inertial fusion energy.

Figures (5)

• Figure 1: (color online) Schematic of cascaded spectral broadening using multiple URPPs, where the laser sequentially passes through each URPP with a random height structure.
• Figure 2: (color online) Laser speckle distributions for (a) $f_m/\omega_0 = 0$, and (b) $f_m/\omega_0 = 0.168\%$, where the horizontal axis represents time $t$ ($fs$) and the vertical axis represents transvers axis $y/\lambda_0$. (c) Statistical histogram of spectral broadening $\Delta\omega/\omega_0$. (d) Comparison of the spectra for $f_m/\omega_0 = 0$ and $f_m/\omega_0 = 0.168\%$.
• Figure 3: (color online) (a) Comparison of spectra with different modulation frequencies, where the blue, green, black, and purple curves correspond to $f_m/\omega_0 = 0, 0.019\%$, $0.037\%$, and $0.056\%$, respectively. (b) Comparison between theoretical and numerical results of broadening bandwidth varying with modulation frequency, where the focal spot size is 650 um.
• Figure 4: (color online) Spatiotemporal speckle distributions for (a) $f_m/\omega_0 = 0$, and (b) $f_m/\omega_0 = 0.168\%$, respectively. (c) Probability density function (PDF) of the normalized intensity for different modulation frequencies. The blue and red curves correspond to $f_m/\omega_0 = 0$ and $f_m/\omega_0 = 0.168\%$, respectively, while the black line shows the theoretical distribution of Rayleigh speckle. (d) Spatial uniformity of laser speckle smoothed by a single URPP with different modulation frequencies.
• Figure 5: (color online) (a) Spectra of the incident laser, where the blue and red curves correspond to lasers smoothed by a URPP with $f_m/\omega_0 = 0$ and $f_m/\omega_0 = 0.043\%$, respectively. (b) Spatiotemporal speckle distributions for the cases of $f_m/\omega_0 = 0$ and $f_m/\omega_0 = 0.043\%$, where $\tau$ is the laser period. (c) Spectra of the scattered lights, with colors consistent with panel (a). (d) Comparison of the electron energy spectra heated by two lasers smoothed by a URPP with different modulation frequencies.