Evan M. Lally

Multiplexed high temperature sensing with sapphire fiber air gap-based extrinsic Fabry-Perot interferometers

In this Letter we present a high temperature multipoint sensing method using sapphire fiber air gap-based extrinsic Fabry-Perot interferometers. Three sensors are fabricated and tested in a single sensing link. Experimental results show that the air gap-based high temperature sensors have a very high temperature sensitivity (>20 nm/ degrees C) and resolution (<0.3 degrees C) and are capable of operating at temperatures well above 1000 degrees C. The multiplexed sapphire sensors present a significant advancement over traditional single-point sensors for critical high temperature applications.

Demonstration of an All-Sapphire Fabry–Pérot Cavity for Pressure Sensing

This letter presents a first demonstration of a monolithic sapphire Fabry-Pérot (FP) cavity for pressure sensing. The prototype was constructed by combining reactive ion etching with direct wafer bonding. Long-term testing proves that the adhesive-free wafer bond is sufficient to create a sealed FP cavity as a pressure transducer. Pressure measurement over a range of 0.04-1.38 MPa has been demonstrated at room temperature using white-light interferometry. With an all-sapphire configuration, the prototype reported here can be further developed into a pressure sensor with high-temperature harsh-environment capability.

Zro2 Thin-Film-Based Sapphire Fiber Temperature Sensor

A submicrometer-thick zirconium dioxide film was deposited on the tip of a polished C-plane sapphire fiber to fabricate a temperature sensor that can work to an extended temperature range. Zirconium dioxide was selected as the thin film material to fabricate the temperature sensor because it has relatively close thermal expansion to that of sapphire, but more importantly it does not react appreciably with sapphire up to 1800 °C. In order to study the properties of the deposited thin film, ZrO2 was also deposited on C-plane sapphire substrates and characterized by x-ray diffraction for phase analysis as well as by atomic force microscopy for analysis of surface morphology. Using low-coherence optical interferometry, the fabricated thin-film-based sapphire fiber sensor was tested in the lab up to 1200 °C and calibrated from 200° to 1000 °C. The temperature resolution is determined to be 5.8 °C when using an Ocean Optics USB4000 spectrometer to detect the reflection spectra from the ZrO2 thin-film temperature sensor.

Fabrication of a Miniaturized Thin-Film Temperature Sensor on a Sapphire Fiber Tip

We report a miniature thin-film-based high-temperature sensor prototype with potential for batch fabrication. A thin Tantalum Pentoxide film was deposited on the tip of a polished sapphire fiber using electron-beam evaporation. Using low-coherence optical interferometry, the film was tested as a temperature sensor from 200°C to 1000°C, and its resolution was measured to be 1.4°C. In addition to its potential for low-cost batch production, the sensors ultra-miniature size of only 75 μm makes it ideal for embedded sensing applications in harsh environments.

Method of multiple references for 3D imaging with Fourier transform interferometry.

This letter presents an improved phase referencing technique, called Method of Multiple References, for optical profilometry. Based on a lookup table, the method eliminates several major drawbacks of single-reference Fourier Transform Interferometry by enabling surface error correction for steep slopes and step discontinuities, and by allowing mapping of multiple discrete objects using a single image set. The algorithm is tested using a fiber optic coupler-based FTI system and shown to have RMS surface error less than 0.03 mm.

Optimization of Single-/Multi-/Single-Mode Intrinsic Fabry–Perot Fiber Sensors

The multiplexing capacity of single-/multi-/single- mode (SMS) intrinsic Fabry-Pérot fiber sensor is dramatically enhanced by reducing its insertion loss. This improvement is achieved by controlling the cavity length to promote refocusing of the guided light when entering the lead-out single mode fiber. With this technique, the round-trip insertion loss of the sensor is reduced from about -3 dB on average to below -0.5 dB, and the signal-to-noise-ratio-defined multiplexing capacity is accordingly increased from six to more than twenty. The paper employs a mode theory based approach to rigorously treat the refocusing problem. Other engineering issues, such as splicing condition and sensor additional phase are analyzed and demonstrated as well.

Toward Eliminating Signal Demodulation Jumps in Optical Fiber Intrinsic Fabry–Perot Interferometric Sensors

Fiber optic Fabry-Perot sensors are commonly interrogated by spectral interferometric measurement of optical path difference (OPD). Spurious jumps in sensor output, previously attributed to noise, are often observed in OPD-based measurements. Through analysis and experimentation based on intrinsic Fabry-Perot interferometric (IFPI) sensors, we show that these discontinuities are actually caused by a time-varying interferogram phase term. We identify several physical causes for varying initial phase and derive a threshold value at which it begins to cause errors in the sensor output. Finally, we present a total phase measurement method as an alternative to OPD-based techniques to reduce the occurrence of output signal jumps.

Sapphire direct bonding as a platform for pressure sensing at extreme high temperatures

Direct bonding between two epitaxy-ready (EPI polished) sapphire wafers is demonstrated as the basis for an all-sapphire pressure sensor. Through chemical processing, hydrogen pre-bonding, and a final high-temperature bakeout, the two single-crystal wafers are directly bonded without the use of any adhesive or intermediate layer. Dicing across the edge of the structure and inspection of the diced pieces with a scanning electron microscope (SEM) indicates a successful direct bond. Control of the bonding wave generates an air bubble sealed between the two bonded sapphire wafers. Optical interference-based measurements of the bubble height and shape at pressures from 0 to 60psig prove that the bubble is sealed by the bonded wafers and demonstrate the potential for sapphire direct bonding as a means of constructing an all-sapphire pressure sensor. Since the structure contains no adhesives, such an all-sapphire sensor is ideal for pressure sensing in extremely harsh, high-temperature environments, potentially operating at temperatures over 1500°C.

Intrinsic Fabry-Pérot interferometric (IFPI) fiber pressure sensor

An optical fiber Single/Multi-/Single-mode Intrinsic Fabry-Pérot Interferometer (SMS-IFPI) pressure sensor has been demonstrated using a silica tube-based pressure transducer hermetically sealed by thermal fusion bonding. The sensor, made entirely of fused silica, contains an IFPI strain sensor enclosed by a CO2 laser-bonded outer tube. A sensor prototype is constructed and demonstrated for single point pressure sensing at high temperature (600°C), with temperature compensation achieved through co-location of an SMS-IFPI temperature sensor. The inline geometry and low transmission loss of the SMS-IFPI sensor makes it suitable for frequency division multiplexing (FDM) in a single fiber branch. In future work, we envision multiplexing of up to eight such IFPI pressure sensors along a single fiber branch for quasi-distributed pressure measurement.

Temperature-Shifted White-Light Interferometry for Equalization of a Fiber Coupler to Near-Zero Path Length Difference

A method for equalizing the path lengths of two arms of an optical fiber coupler is presented as a critical step towards construction of a high-resolution 3-D interferometric imaging system. Based on white-light interferometry (WLI), the technique combines absolute measurement capability with the ability to accurately measure near-zero path length differences. A controlled temperature increase in one arm of the coupler generates a shift in the WLI measurement, allowing the fibers to be polished to near-equal lengths. The technique is demonstrated to equalize the fiber lengths to better than 0.6 mum.