Fabrication of silicon-tipped fiber-optic temperature sensors using aerogel-assisted glass soldering with precise laser heating

Different fiber-optic sensors have different configurations. Specifically, those with silicon diaphragms attached to the fiber tip have drawn significant research attention because they function like a Fabry-Perot interferometer (FPI). Besides, they exhibit high measurement speeds and high sensitivity due to the large thermal diffusivity and the large thermo-optic coefficient of silicon and the small size of the sensing element. Conventional fabrication of these sensors involves direct deposition of very thin polycrystalline silicon film layers onto fiber end using various vacuum deposition techniques like sputtering. Although this approach endows the sensors with high-temperature capability, the resulting silicon FPI has a short cavity length, leading to low-temperature resolution.

Bonding methods have emerged as promising techniques for overcoming the drawbacks of vacuum deposition to aid the development of fiber optic sensors with desired properties. Unfortunately, different bonding methods have different limitations that hinder their practical applications. For example, UV epoxy produces significantly lower temperature capability of the sensor while direct fusion bonding has stringent diameter requirements and the process induces high fusion temperature that is difficult to control.

To overcome these limitations, new bonding techniques have been proposed. Among them, glass soldering has been identified as a potential candidate. The ability to tailor the melting temperature of glass powders by altering their composition endows the fabricated sensors with a variety of operating temperatures. It also has high-temperature resistance, high stability and large tolerance to different diaphragm sizes. . Unfortunately, it has not been studied for the purpose of fabricating silicon-based fiber optic sensors.

On this account, Dr. Qiwen Sheng, Dr. Nezam Uddin, Mr. Bohan Zhou, Ms. Xiaoli Wang and Professor Ming Han from Michigan State University developed a new fabrication method of silicon-tipped fiber optic FPI temperature sensors by bonding either a coated or uncoated small silicon diaphragm to the tip of the fiber using aerogel-assisted glass soldering with precise 980 nm laser heating. An optical fiber was used to deliver the heating laser to the silicon FPI, while a 1550 nm white light system was utilized to monitor the FPI temperature changes, enabling localized, precise laser heating and precise temperature control. Silica aerogel plates were used as substrates for silicon parts. The work is currently published in the journal, Optics Letters.

The research team showed that aerogel substrates significantly improved the heating efficiency by significantly reducing the thermal heat loss from the bonded parts to the external environment. Unlike other substrates, the aerogel substrate allowed the fabricated sensor to achieve a temperature higher than the melting point of the glass powders desirable for bonding with relatively small laser heating. This method was validated by fabricating and characterizing the temperature capability and measurement resolution of low-finesse and high-finesse FPI sensors prepared using glass powders with different melting points. The low-finesse sensors fabricated at 900 °C exhibited a good temperature resistance and 0.35 mK resolution, while the high-finesse sensor fabricated at 400 °C exhibited a 0.07 mK temperature resolution.

Carrying out the aerogel-assisted glass soldering based bonding process in an open space at room temperature instead of a confined space was recommendable to achieve convenient and desired optical alignment of the sensor. The precise temperature control was of great importance in minimizing the perturbation of the optical alignment and reducing the possibility of inducing thermal damage to the optical parts during bonding. Furthermore, glass powders with a wide range of melting point are highly flexible and can accommodate different coating materials used in forming silicon FPIs.

In summary, Michigan State University scientists successfully developed a breakthrough technology in the fabrication of silicon-tipped fiber-optic sensors. Based on the new approach, low- and high-finesse FPI sensors with good performance can be fabricated. The remarkable properties of the silica aerogel, including small thermal conductivity, thermal diffusivity, high melting temperature and specific heat capacity, make it suitable for fabricating high-temperature sensors. In a statement to Advances in Engineering, Professor Ming Han, the lead and corresponding author said that aerogel-assisted soldering coupled with laser heating is a promising method for high-precision fabrication of fiber optic sensors.