Frequency Modulation Self-Mixing Interferometer: Architecture and Performances

Introduction
In recent years, a new type of optical interferometer has been developed, with the main application as a vibration sensor, based on the well-known back-injection technique (known as self-mixing interferometry, SMI), but measured in its frequency modulation (FM). SMI has already been demonstrated in different sensing applications, ranging from flow measurements to vibration, distance, or speed measurements. Frequency modulation in an optically back-injected laser provides a measurement signal with a signal-to-noise ratio about two orders of magnitude higher than amplitude modulation, bringing this type of interferometer, which normally has limited performance, to the level of much more complex and expensive interferometric techniques. The trade-off concerns the mechanical and electronic complexity of the setup, as it requires a frequency-amplitude discriminator, typically the addition of another interferometer (Mach-Zehnder), which must be kept hooked in quadrature.

Methods
This work describes the architecture of the FM-SMI, with particular emphasis on how to realize the electronics required for the various functions: laser diode power supply, high frequency transimpedance, and feedback control loop to keep the Mach-Zehnder interferometer in the correct working point.

Results
The realized interferometric sensor shows remarkable performance, with a noise equivalent displacement (NED) of about, depending on the laser source linewidth and the feedback coefficient C.

Conclusion
This work shows how to realize an FM-SMI, applicable to the various sensor applications for which the self-mixing interferometer is used, but with significantly superior measurement performance, in terms of bandwidth, sensitivity, and NED.