We have developed and demonstrated a new method to calibrate laser Doppler vibrometers (LDVs). The method uses two acousto-optic modulators to frequency shift the light from the LDV under test by a known quantity to create a synthetic velocity shift that is traceable to a time and frequency reference. We have results demonstrating the reduction of the method to practice for discrete velocity shifts and for sinusoidal velocity shifts that would be equivalent to what would be observed in a calibration that follows ISO Standard 16063-41, which is based on a comparison-type measurement to a laser homodyne interferometer that is defined as the primary standard, resulting in the LDV as being considered as a secondary calibration. In contrast, our method enables a traceable primary method for the LDV. The method also enables the user to simulate arbitrary mechanical excitation conditions to test and calibrate the LDV, including sweeping the synthetic vibration excitation frequency to directly characterize the bandwidth of an LDV together with its associated electronics. We are now designing a fiber optic coupled version of the system that we believe will make the device easier to use and more portable.
Our results show that this approach is immediately useful as a tool for characterizing the bandwidth of an LDV as a system together with its acquisition and control electronics and amplifiers. Our future work is focused on further improvements on the system design and carrying out a full uncertainty analysis. One improvement in the design may be to measure the frequency shift with a photodiode rather than from the signal generators driving the AOMs. This could capture any offsets or fluctuations between the drivers and the light entering the LDV, e.g. due to refractive index fluctuation. Another improvement may be to move the collimator before the beam splitter.