1 January 2002

Interfering Light

By Wade Pulliam and Patrick Russler

Micromachined silicon carbide-sapphire fiber-optic pressure sensor operates in 3,600°F environment.

Acquiring accurate, transient measurements in harsh environments has always taxed the available measurement technology. Until recently, the technology to directly measure certain properties in extremely high-temperature environments has not existed.

Advancements in optical measurement technology have led to the development of measurement techniques for pressure, temperature, acceleration, and skin friction using extrinsic Fabry-Perot interferometry (EFPI).

The basic operating principle behind EFPI enables the development of sensors that can operate in the harsh conditions associated with turbine engines, high-speed combustors, and other aerospace propulsion applications where the flow environment is dominated by high-frequency pressure and temperature variations caused by combustion instabilities, blade-row interactions, and unsteady aerodynamic phenomena.

Using micromachining technology, these sensors are quite small and therefore ideal for applications where restricted space or minimal measurement interference is a consideration.

LIVE IN BENIGN ENVIRONMENT

Although sensors using current technologies provide measurement range, frequency response, and most other requirements for aerospace applications, these sensors perform well only in benign environments. There are at least two operating regimes where current technology fails to meet minimum requirements: environments with either high temperatures and/or high electromagnetic interference. Harsh environmental conditions in aerospace applications constitute an extreme in at least one of these two conditions.

As engine designers continue to push the limits of current propulsion technology, the need to monitor flow conditions in harsh environments for performance evaluation and control of new propulsion systems becomes paramount. Because the flow environment inside an operating propulsion system can be harsh, the demands placed on sensor and instrumentation technology are considerable.

In order to meet extreme design goals, engine designers will need health monitoring and engine test instrumentation that can withstand temperatures as high as 1,300°C (2,372°F), while providing adequate frequency response and low flight weight. Control methodologies developed to avoid high-cycle fatigue, combustion instabilities, or compressor surge will require sensors capable of operating in the engine environment.

Currently available technology cannot accommodate this combination of high-temperature tolerance and high-frequency response. These and other propulsion programs have created a clear need for accurate, high-frequency sensors capable of operating in harsh environments.

PROPAGATE AIR GAP

To many researchers, fiber-optic sensors seem to be the best available technology for acquiring measurements in these harsh environments. These sensors have begun to replace conventional electrical sensors in specific applications.

The transducing technique used by fiber-optic sensors does not involve electrical signals, so they are essentially immune to electromagnetic interference. These sensors also function at higher temperatures than their electrical counterparts.

However, the modest increase in operating temperature that fiber-optic sensors afford rarely merits their additional cost. For fiber optics to compete with other sensor technologies, the increased temperature tolerance these sensors offer must be significantly higher to justify their added expense.

Efforts to address this issue are at the forefront of current fiber-optic sensor research.

A variety of fiber-optic sensing techniques have been put to use in the past two decades, including intensity-based interrogation and interferometry. One versatile technique for fiber-optic sensor applications is EFPI.

EFPI-based sensors use a distance measurement technique based on the formation of a low-finesse Fabry-Perot cavity between the polished end face of a fiber and a reflective surface.

Light passes through the fiber, after which a portion of the light reflects off the fiber/air interface (R1). The remaining light propagates through the air gap between the fiber and the reflective surface and reflects back into the fiber (R2). These two light waves interfere constructively or destructively based on the path length difference traversed by each.

The path length modulates the interaction between the two light waves in the Fabry-Perot cavity. The resulting light signal then travels back through the fiber to a detector, where the signal demodulates to produce a distance measurement. Several demodulation methods exist to convert the return signal into a distance measurement.

EFPI IN PRESSURE SITUATION

The basis of a fiber-optic pressure sensor using this technology has an optical fiber and glass tube fiber spacer bonded in the center opening of the sensing element. The optical gap between the bottom of the diaphragm and the end face of the fiber is a Fabry-Perot cavity.

Light interference resulting from the internal reflection of light at the fiber end face (R1) and the reflected light off of the bottom surface of the diaphragm (R2) translates to an optical path length. The optical gap varies with diaphragm deflection, which in turn varies with applied pressure.

The reference port on the bottom of the sensing element acts as a vent through which air passes to maintain a constant pressure on the reference side of the diaphragm.

Optical fiber pressure sensors that utilize EFPI have exceptional tolerance to elevated temperatures. Current EFPI-based fiber-optic sensors are capable of sustained operation up to approximately 600°C (1,100°F). This limitation exists due to the small number of materials that can withstand higher temperatures and still exhibit the required mechanical properties for sensor operation.

For example, silicon often serves as a diaphragm material for EFPI-based pressure sensors and accelerometers. The melting point of silicon is 1,415°C. But it begins to deform at a much lower temperature, making the useful mechanical limit of the material somewhere between 550°C and 600°C (1,025°F to 1,100°F).

Another limitation is the operating temperature of the silica fiber, generally quoted as 900°C (1,650°F), above which the core and the cladding material begin to migrate. Extending the operating temperature range of EFPI-based sensors will happen by using materials with higher thermoplastic and/or melting points.

Research groups are experimenting with more temperature-tolerant materials. Replacing silica fiber with sapphire fiber seems a viable method of increasing the temperature range of optical fibers for several years. Sapphire has excellent optical qualities, with a melting point over 2,000°C (3,600°F). Work has begun replacing silicon as the sensor's structural material with silicon carbide (SiC). SiC has an operational temperature of over 1,100°C (2,000°F).

Combining a SiC sensor with sapphire optics, one can fabricate a pressure sensor to withstand the harshest turbomachinery environment. IT

Behind the byline

Wade Pulliam has a Ph.D. in aerospace engineering from Virginia Tech. He is a research faculty member at UCLA in the Mechanical and Aerospace Engineering department.

Patrick Russler has a master's in mechanical engineering from Virginia Tech. He works for Applied Research Associates, Inc. in Raleigh, N.C., where he develops engineering simulation software tools.

Read the original ISA 2001 paper.