Performance characteristics of a new generation pressure microsensor for physiologic applications

Abstract

A next generation fiber-optic microsensor based on the extrinsic Fabry-Perot interferometric (EFPI) technique has been developed for pressure measurements. The basic physics governing the operation of these sensors makes them relatively tolerant or immune to the effects of high-temperature, high-EMI, and highly-corrosive environments. This pressure microsensor represents a significant improvement in size and performance over previous generation sensors. To achieve the desired overall size and sensitivity, numerical modeling of diaphragm deflection was incorporated in the design, with the desired dimensions and calculated material properties. With an outer diameter of approximately 250 microm, a dynamic operating range of over 250 mmHg, and a sampling frequency of 960 Hz, this sensor is ideal for the minimally invasive measurement of physiologic pressures and incorporation in catheter-based instrumentation. Nine individual sensors were calibrated and characterized by comparing the output to a U.S. National Institute of Standards and Technology (NIST) Traceable reference pressure over the range of 0-250 mmHg. The microsensor performance demonstrated accuracy of better than 2% full-scale output, and repeatability, and hysteresis of better than 1% full-scale output. Additionally, fatigue effects on five additional sensors were 0.25% full-scale output after over 10,000 pressure cycles.

FIGURE 1

Schematic of the EFPI measuring technique. (a) Light propagates through an optical fiber, a portion of the light is reflected by the polished end face of the fiber (R1) and the remaining light travels through the Fabry–Perot cavity and is reflected back by a diaphragm (R2). The optical path length changes as pressure deflects the diaphragm and can be determined through interferometric measurements of R1 and R2. (b) Resultant interferogram based on the reflections of R1 and R2 show the characteristic fringe pattern, with a unique frequency component that increases as the optical path length increases.

FIGURE 2

Magnified image that shows a human hair compared to the pressure sensor mounted at the end of a 135 µm diameter single-mode optical fiber.

FIGURE 3

Comparison of theoretical deflections of the microsensor diaphragm compared to a representative calibration curve of the described sensor. The sensor demonstrated significantly greater sensitivity over the dynamic range of 0–250 mmHg compared to the theoretical deflection of a rigidly supported diaphragm. However, the model for a simply supported diaphragm shows significantly greater deflection than the sensor response. Design of the sensor leads to a hybrid of a simply supported and rigidly support diaphragm.

FIGURE 4

Typical calibration curves for the pressure microsensor showing the deflection of the diaphragm over 250 mmHg before and after 1000, and 10,000 cycles of a 0 and 300 mmHg square wave. Each curve represents the average of two cycles of pressure stepped up from 0 to 250 mmHg and then back to 0 mmHg. Error bars of ±1 standard deviation are shown. A fourth-order polynomial best fits the calibration data. Additional residual analysis demonstrated that there were not any effects in sensor response over the repeated cycling.