Consequently, the laser frequency profile is much more stable with the high-speed mirror. At the peak mirror velocity the laser modulation exceeds the Nyquist limit of the data acquisition system by about 800 kHz and is folded back to lower frequency. The noise in the frequency profile is significant; perhaps in consequence, velocity fluctuations caused by vibrations are not visible. A portion of the noise may be caused by refractive index fluctuations resulting from turbulence in front of the mirror.