Hybrid optical sensors for extreme environments

The sensor community is constantly challenged to provide robust solutions for the extreme conditions experienced in space stations, aircraft and ship propulsion (such as jet engines), nuclear and fossil-fuel electric power plants, industrial-materials production plants (e.g., steel fabrication, corrosive chemical-liquid factories), machined-parts design and testing platforms, and hostile military-target acquisition/recognition systems.

We recently proposed a hybrid-optics approach to robust sensor design that combines the secure transmission capability of optical fibers with a minimally invasive, laser-targeted light beam to produce the required optically encoded signal. The free-space laser beam provides the physical isolation between the extreme-environment and the friendlier signal-monitoring zones, while standard fiber-optical routing offers secure light delivery and capture for sensing and remote post-processing. In addition, we added electronically agile beam forming optics to condition the transmit/receive beam, thus adding intelligence and robustness to the spatial-sensing operations. Depending on the application, these hybrid sensors can use an appropriate optical detection method such as blackbody-radiation capture, laser interferometry, tunable and broadband optical-spectrum analysis, or multidimensional spatial-light processing. To date, we have reported design and experimental results for harsh-environment sensing of hot^1–6 and cold temperatures,⁷ high pressure,^8,9 solid-object range,¹⁰ liquid level,¹¹ and 3D solid-object shape.¹²

As an example, our first hybrid sensor is an extreme-environment temperature probe. Accuracy, reliability, and long lifetimes are critical parameters for sensors measuring temperatures in gas turbines of clean, coal-fired power plants. Greener, high-efficiency, next-generation power plants need gas turbines that operate at very high temperatures of ~1600^°C, where present thermocouple temperature-probe technology fails to deliver reliable and accurate readings over long lifetimes. To solve this pressing problem, we developed a new hybrid class of all-silicon carbide (SiC) optical sensors. A single-crystal SiC optical chip is embedded in a sintered SiC-tube assembly, forming a coefficient-of-thermal-expansion (CTE)-matched all-SiC front-end probe. Because chip and host material are CTE matched, optimal handling of extreme thermal ramps (>1000^°C in a few seconds) and temperatures (e.g., 2000^°C) is possible. Light is routed via fibers from the post-processing zone to the entry point of the sensing inlet where a laser beam is generated within the sensor probe (see Figure 1). Its tip is located in the hot gas of the combustion chamber.

Figure 1. Basic design of the all-passive front-end all-silicon carbide (SiC) temperature probe with its active motion-control backend. SMF: Single-mode fiber.

The probe consists of three connectable parts (see Figure 2). First, a long (42cm) all-SiC hollow tube with one end open and the other containing the embedded, thick (400μm) SiC optical chip responds optically and mechanically to the temperature of the gas. Second, a steel pressure connector engages with the probe's open end to form a pressured, sealed connection. Third, a probe window assembly with a vacuum valve generates a weak vacuum inside the probe cavity. The assembled probe is inserted into the gas-turbine combustor inlet using a pressure fitting. A short-range (<20cm) optical-transceiver module provides the laser beam for SiC-chip targeting and temperature-encoded receive-beam capture. Unlike thermocouple and optical-fiber approaches that require extra protection tubes around the electrical/optical wires, our probe does not require such special protection. Hence, we avoid CTE mismatch problems as well as the need for complex, long-term packaging of many protection layers.

Figure 2. All-SiC temperature front-end probe. (a) Unassembled. (b) Assembled.

We recently completed the first successful industrial-combustor rig test of our new front-end sensor, demonstrating structural robustness to 1600^°C. We tested over a one-month duration, using eight combustor thermal-shock tests. Figure 3 shows a mechanical structural-integrity test conducted with an oxy-acetylene flame where the probe remains intact but the thermocouple melts.

Figure 3. All-SiC probe and high-temperature thermocouple under oxy-acetylene flame thermal and localized thermal-ramp joint test with temperatures reaching 1600^°C.

In summary, we have combined a number of separate innovations to develop a powerful hybrid-sensor design that couples some of the best features of fiber and free-space optics, lasers, thermal radiation, spectral content, optical refractive-index changes, mechanical deformations, and spatial processing to deliver a robust sensor suited for extreme environments. The synergy of reliable sensor materials and sound optical engineering and signal processing promises a better future for the challenging sensing demands of the 21^st century. Our future technical work will optimize probe design for front-end SiC-chip long-lifetime operations and combined temperature and pressure measurements. In addition, we expect to collaborate with appropriate industrial partners for advanced engine testing.