Understanding the cytoarchitecture and wiring of the brain requires improved methods to record and stimulate large groups of neurons with cellular specificity. This requires miniaturized neural interfaces that integrate into brain tissue without altering its properties. Existing neural interface technologies have been shown to provide high-resolution electrophysiological recording with high signal-to-noise ratio. However, with single implantation, the physical properties of these devices limit their access to one, small brain region. To overcome this limitation, we developed a platform that provides three-dimensional coverage of brain tissue through multisite multifunctional fiber-based neural probes guided in a helical scaffold. Chronic recordings from the spatially expandable fiber probes demonstrate the ability of these fiber probes capturing brain activities with a single-unit resolution for long observation times. Furthermore, using Thy1-ChR2-YFP mice we demonstrate the application of our probes in simultaneous recording and optical/chemical modulation of brain activities across distant regions. Similarly, varying electrographic brain activities from different brain regions were detected by our customizable probes in a mouse model of epilepsy, suggesting the potential of using these probes for the investigation of brain disorders such as epilepsy. Ultimately, this technique enables three-dimensional manipulation and mapping of brain activities across distant regions in the deep brain with minimal tissue damage, which can bring new insights for deciphering complex brain functions and dynamics in the near future.

The Letter reports the inscription of fiber Bragg gratings (FBGs) in a single-crystal sapphire optical fiber via a point-by-point method by 780 nm infrared-femtosecond laser pulses. Compared to phase mask exposure, the use of the point-by-point method for the inscription provides a flexible way to fabricate sapphire FBGs and to make wavelength division multiplexing in sapphire fiber more practicable. The multiplexing of three cascade gratings is demonstrated, and their performance up to 1400°C is tested. The permanent enhancement of reflectivity by a factor of about 5 after heat treatment and the nearly linear temperature response with a slope of 25.8 pm/°C are demonstrated.

This Letter introduces a fiber Bragg grating (FBG) in a micro-single-crystal sapphire fiber (micro-SFBG) for sensing applications in high-temperature and harsh environments. The FBG was fabricated by a point-by-point method via an IR-femtosecond laser in a large-diameter sapphire fiber that was then wet-hot acid etched to achieve microfiber size, which culminated in fabricating and characterizing a 9.6 μm diameter micro-SFBG. The refractive index measurement ranging from 1 to 1.75 and temperature measurement from room temperature to 1400°C are also reported.

Readily available temperature sensing in boilers is necessary to improve efficiencies, minimize downtime, and reduce toxic emissions for a power plant. The current techniques are typically deployed as a single-point measurement and are primarily used for detection and prevention of catastrophic events due to the harsh environment. In this work, a multi-point temperature sensor based on wavelength-multiplexed sapphire fiber Bragg gratings (SFBGs) were fabricated via the point-by-point method with a femtosecond laser. The sensor was packaged and calibrated in the lab, including thermally equilibrating at 1200 °C, followed by a 110-h, 1000 °C stability test. After laboratory testing, the sensor system was deployed in both a commercial coal-fired and a gas-fired boiler for 42 days and 48 days, respectively. The performance of the sensor was consistent during the entire test duration, over the course of which it measured temperatures up to 950 °C (with some excursions over 1000 °C), showing the survivability of the sensor in a field environment. The sensor has a demonstrated measurement range from room temperature to 1200 °C, but the maximum temperature limit is expected to be up to 1900 °C, based on previous work with other sapphire based temperature sensors.

High temperature sensor with robust performance, minimum footprint, and fast response is desirable for many applications in extreme environments. This article introduces a miniature high temperature sensor based on fiber Fabry-Perot interferometer. The interferometric cavity is directly fabricated on the tip of a 125 μm single-crystal sapphire fiber through femtosecond laser micromachining and CO ₂ laser welding with the sapphire fiber as a light-guiding element. The sensor has an all-sapphire structure to ensure both physical and chemical stability. The sensor was tested in the lab from room temperature up to 1455 °C and shows an excellent temperature response with 1.32 nm/ °C at room temperature and full-range average resolution of 0.68 °C. The thermal response of the sensor is evaluated via both numerical simulation and experiment. Owing to the small size and fast thermal diffusivity of sapphire, the thermal response time of ~1.25 ms has been experimentally demonstrated.