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Experiments for Improving Fiber Optic Sensors to Commence Soon

With the aim of improving the accuracy and sensitivity of infrasonic optical sensors for long distances, two researchers from the University of Alabama in Huntsville (UAH) aim to study the characteristics of these sensors.

Dr. Qiuhai Ken Zuo, left, and Dr. Lingze Duan plan experiments that could make infrasonic optical sensors more sensitive and accurate over long distances. (Credit: Michael Mercier | UAH)

Their study is a combination of theoretical modeling with experiments, and it has the potential to change the operation of the sensors across a variety of applications - from national security to earth system science - in the years to come.

National security is one of the main areas where fiber optic infrasound sensors are used. Sensors that function under the level of 20 Hz are used to evaluate changes in the geological characteristics that are caused due to earthquakes, glacier movements and volcanic activities, and to keep an eye on global nuclear weapon tests. Fiber optic sensors are also commonly used in devices employed to detect minor shifts in the material structural properties over long distances. Some instances include using these sensors to keep tabs on the conditions of structures like skyscrapers and bridges.

The National Science Foundation (NSF) Electronic, Photonic and Magnetic Devices Program has issued a grant of $340,314 to Dr. Lingze Duan and Dr. Qiuhai Ken Zuo, associate professors of physics, and mechanical and aerospace engineering, respectively.

The researchers will be conducting ultra sensitivity tests on fiber optic sensors at low frequencies; these tests are the first of their kind to be conducted in a building that has been designed for the purpose of keeping external disturbances from affecting the sensors, the Optics Building in UAH. Cross-College Faculty Research funds from UAH’s Office of the Vice President for Research and Economic Development supported the work of the duo from 2013 to 2015.

Most fiber-optic sensors generally use an optical technique known as inference to explore the characteristics of interrogating light. These characteristics, which include phase and intensity, are ideally dependent on the external physical quantities, such as stress, temperature and pressure, to be measured. However, during practical use, the sensors may impact changes to the properties of light. This in turn makes the measurement outcomes ambiguous and reduces the sensitivity of the sensors.

We will explore the fundamental physics that set the ultimate limit of optical fiber sensors. With any fiber sensor, there is an intrinsic limitation of sensitivity. It is caused by spontaneous fluctuations inherent in the fiber and is governed by fundamental thermodynamic laws. In order to make very sensitive fiber optic sensors, we need to understand the physics underlying these spontaneous fluctuations.

Dr. Lingze Duan, Associate Professor, UAH

Low-frequency noise is of special interest to the researchers at UAH. As the wavelength of low-frequency waves are longer, they can travel longer distances. This enables the sensors that operate under the range of 20 Hz frequency to detect changes that occur over large distances.

Despite this, scientists were unable to discover the reason behind the failure of model sensors set below 1 kHz to function under the theoretical noise parameters fixed by a thermodynamic model, when they function properly at higher frequencies. Excessive noise in the laser signal inside the optic fiber was encountered when the frequency is low, affecting the accuracy of the sensors.

In order to demonstrate these discrepancies, Dr. Duan created and presented a thermomechanical model in 2010. A couple of years after publishing his first paper, he presented a theoretical paper to reconcile his previous model with a thermodynamic model.

According to thermodynamic laws, a macroscopic body at non-zero temperatures experiences microscopic fluctuations due to the random thermal motion of the constituent atoms and molecules. Such microscopic fluctuations cause spontaneous variations of local properties, such as local temperature and local strain, in the macroscopic body and induce spontaneous noise when the macroscopic body interacts with other media such as waves and particles. For example, the thermodynamic noise in optical fibers is closely related to spontaneous fluctuation of local temperatures, while the thermomechanical noise is linked to random motion of microscopic defects.

Dr. Lingze Duan, Associate Professor, UAH

Duan, however, understood that the issue cannot be resolved only with theoretical effort.

“People were debating whether the thermomechanical model can explain their experimental data. But the reality is, there has not been any experiment that definitively shows how much spontaneous noise optical fibers generate at infrasonic frequencies,” Dr. Duan says. “What we really need is experimental work specifically designed to probe the low-frequency noise in optical fibers.”

Duan’s proposal to the NSF contained the framework of an experiment that makes use of a new hybrid sensor configuration in order to attain, what he terms as, an unprecedented sensitivity level in the infrasonic area.

“If this experiment succeeds, not only is it a significant advance in the understanding of the fundamental limit of fiber optic sensors, but the experimental technique will also offer a blueprint for future ultra-sensitive fiber optic infrasound sensors,” says Dr. Duan.

In order to take his research from it experimental stage, Duan collaborated with computational mechanics and materials modeling expert, Dr. Zuo. Together, the two researchers created a 3D model for thermomechanical noise.

My role in this project is to improve the fidelity of the one-dimensional model by modeling the three-dimension behavior of the fibers, including the lateral inertia and bending modes of deformation, in addition to the longitudinal modes considered in the one-dimensional model. There are some artifacts in the one-dimensional models that are not present in the data.

Dr. Qiuhai Ken Zuo, Associate Professor, UAH

Dr. Zuo further adds that a polymer coating and glass core are used to make fiber optic cables and when subjected to dynamic disturbance, they react in a 3D and visco-elastic manner.

“We plan to develop a theoretical model for thermomechanical noise based on the three-dimensional response of the glass and the polymer coating. I will bring in some modeling tools for wave propagation in visco-elastic materials, and that is where mechanical engineering becomes a part of it.”

Dr. Zuo says that conversation had during lunchtime led to the collective effort.

“Initially, we started a conversation just between two colleagues, and then we got those Cross-College Faculty Research grants that provided us the motivation to develop a formal proposal to NSF,” Dr. Duan says. “I thought it was especially beneficial that we could combine the strengths of the colleges of science and engineering.”

A new frequency locking system is being designed by Dipen Barot, a graduate student of Dr. Duan. This system will help in stabilizing the interrogating lasers for the experiments that are to be conducted. The acquisition of the equipment required for the experiment will being in the month of August, 2016.

We want to measure this extremely small, low frequency noise. There are a lot of scientific and engineering challenges that we need to address, for example, thermal and acoustic isolations, laser noise reduction, noise discrimination, etc. It wasn’t done in the past, because it is extremely difficult. But we are confident we have the best chance ever.

Dr. Lingze Duan, Associate Professor, UAH

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