We have developed a new method for laser interferometry that uses a pulsed laser to transform high frequency (GHz) vibrations into a low frequency (MHz) optical signal that can be measured easily with a low noise photodetector. This method has been shown to have a noise floor that is 5 times lower than found in conventional approaches (e.g., continuous wave heterodyne interferometry). Additionally, it is capable of mapping the resonance frequencies and spatial mode shapes with diffraction-limited resolution. Applications in cavity optomechanics and mobile communications will benefit from this method since high frequency mechanical resonators are critical in these areas.
Our invention is a new method for laser interferometry that uses a pulsed laser. The method is capable of measuring vibrations and traveling acoustic waves that are as small as tens of femtometers in amplitude and beyond 10 GHz in frequency. The method synchronizes the pulses of the probe laser with the measured vibrations, such that the driven vibrations of a micro- or nanomechanical structure are at a frequency that is near an integer multiple of the laser pulse repetition rate. The vibrations can then be observed by locking to the beat note between the laser pulses and drive signal. The lock-in process is directly linked to the RF frequency comb found in the laser pulses, making the measurement process very stable. The optical interference signal is the result of an optical mixdown process, yielding a measurement that is orders of magnitude lower in frequency than the actual vibrations. As a result, the challenges in measuring vibrations at gigahertz frequencies, such as the higher noise in gigahertz detectors, and the unavoidable insertion loss and electromagnetic interference at microwave frequencies, are completely avoided by our approach. The method is described in more detail in the attached extended abstract, which was published on May 21, 2018.