The vibrations can be clearly seen in the laser frequency (or optical velocity) profile. The underlying sinusoidal velocity profile can also be clearly seen. The x-axis is monotonic in time, but is itself modified by the velocity variation because it is indexed by zero-crossing. The peak frequency here is 130 kHz and the fluctuations are about 20 kHz. This corresponds to a velocity of 6.5 cm/s with fluctuations of 10% (ignoring the underlying sinusoidal profile). In spite of these velocity variations the SNR of the recorded spectrum is quite good. This indicates that the use of the reference laser to compensate for velocity variation is effective. The high-speed mirror is mounted much more securely on a rugged motor.