Hybrid Interferometer Achieves Record-breaking Strain Resolutions

Researchers at the University of Alabama in Huntsville (UAH) have invented an ultrahigh-resolution interferometer that is sensitive enough to detect weak acoustic signals that are too faint to be picked up by other sensor types. The interferometer is based on a hybrid design that combines the advantages of a double-path configuration with the benefits of optical resonators.

Nabil Md Rakinul Hoque, who is credited with the invention, embedded an optical resonator-based interferometer — the Fabry-Perot type — into a double-path interferometer — the Mach-Zehnder type — to create the device, known as the Mach Zehnder-Fabry Pérot (MZ-FP) interferometer.

Resonator-based interferometers, like the Fabry-Pérot type, allow specific resonance frequencies to pass or reflect from the interferometer. They produce an ultralong optical path length despite their compactness due to the high reflectivity of their mirrors, which establishes a measurable interference pattern between light streams.

A second type of interferometer is based on a common-path or double-path configuration; its sensitivity depends on the length of its arm, which can exceed tens or even hundreds of meters, making the devices prone to bulkiness. Mach-Zehnder and Michelson interferometers are examples of traditional double-path interferometers.

The hybrid scheme of the MZ-FP interferometer allowed the researchers to combine a traditional double-path configuration with fiber optic resonators. Hoque and his colleagues developed a compact interferometric fiber sensor operational at the thermal noise level while being interrogated by an off-the-shelf commercial diode laser.

“The main feature of the new interferometer is its unprecedentedly high signal resolution,” Hoque said.

The research team used identical fiber Fabry-Pérot interferometers as optical-path multipliers along with a soil-based insulation system to enable the MZ-FP interferometer to reach record-breaking strain resolutions across a range of frequencies. In tests, the MZ-FP interferometer achieved 1 femto-strain of resolution, demonstrating the capability to detect the change of 1 billionth of a μm (10⁻⁶ m) out of 1 m.

According to the team, atto-strain resolutions with the MZ-FP could be attainable within the ultrasonic frequency range if the interferometer is scaled-up appropriately.

Lingze Duan, a professor at the university, said that the interferometric fiber sensor set resolution records across a broad frequency span, from the infrasonic range to the ultrasonic range.

Nabil Md Rakinul Hoque’s interferometer design combines the advantages of two types of interferometers that are currently available. The hybrid instrument is compact and highly sensitive, and it supports use in a variety of biomedical and physical fields. Courtesy of Lingze Duan/UAH.

The newly demonstrated device’s aptitude for detecting ultraweak signals opens possibilities for use to make predictions of environmental events, monitor weapons, and offer detection capabilities for climate change research, Hoque said.

Additionally, the optical sensors based on the MZ-FP interferometer could be used to assist acoustic medical diagnoses. “For example, acoustic sensors based on our hybrid interferometer may be able to pick up very weak physiological acoustic signals that reveal human health conditions,” Hoque said, noting that these signals can be too faint to detect with current sensors.

“The most important impact of this work, in my opinion, is that it lays out a feasible path toward reaching unprecedented levels of strain resolutions for passive fiber sensors,” Duan said. “Such a level of sensing resolution will allow fiber optic sensors to pick up much weaker signals than they can right now and greatly broaden the application of fiber optic sensors.”

The research was published in Scientific Reports (www.doi.org/10.1038/s41598-022-16474-y).