Fiber optic interferometric sensors have wide application and may be used to sense, for example, temperature, strain, pressure, vibration, and acoustic waves. One example of a fiber optic interferometric sensor is an Extrinsic Fabry-Perot interferometric (EFPI) sensor, which is based on a combination of two light waves as described in U.S. Pat. No. 5,301,001 to Murphy et al., which is incorporated by reference herein. A typical EFPI sensor consists of a Single-mode input fiber and a reflector fiber aligned by a hollow core silica tube. The operation of such an EFPI can be approximated as a two beam interferometer. When the light arrives at the source fiber end-face, a portion is reflected off the fiber/air interface (R1) and the remaining light propagates through a cavity referred to as an air gap with a second reflection occurring at the air/fiber interface (R2). The distance between R1 and R2 is same as the length of the air gap and is one half of an optical path length (defined below). In an interferometric sense, R1 is the reference reflection, and R2 is the sensing reflection. These reflective signals interfere constructively or destructively based on wavelength and the optical path length between the reference and sensing fibers. Small movements from environmental or other physical forces cause a change in the cavity or gap length causing a phase difference between the sensing and reflecting waves producing fringe-based changes in the intensity. The sensitivity to changes in optical path length is proportional to the visibility of the interference fringes reflected back into the input fiber.
Fiber optic sensing systems extract or demodulate measurement information from the sensor(s) that can then be used to measure the optical path length or indicate relative changes in the optical path length, which translate into sensed changes in temperature, strain, pressure, etc. This extraction of sensor measurement information is referred to here as demodulation. Various demodulation techniques to obtain fiber optic sensor measurement data suffer from their own different, specific, shortcomings. Wavelength-based fiber optic sensor demodulation approaches are slow, expensive, and usually limited to one sensor channel. Intensity-based fiber optic sensor demodulation approaches can be faster and less expensive, but they usually do not permit accurate and absolute measurement. And their cost is generally linear with channel count, resulting in high cost for sensor systems that use a large number of sensors.
These fiber optic sensor demodulation approaches also suffer from component-based errors and other inaccuracies caused for example by environmental conditions. One way to reduce these errors is to use components of very high quality with very tight specifications. But the tradeoff is higher cost. Environmental conditions, as a practical matter, may not be readily controlled, or the cost to control them too high.
Certain demodulation approaches have inherent limitations. For example, if a single wavelength of laser light is used in the sensing system, then the demodulation technique is not reliable for larger gap changes or for absolute measurements. The intensity detected by a one wavelength demodulation system can be identical for two very different gaps. A two wavelength measurement approach also suffers from these limitations.
Current demodulation techniques are also vulnerable to light intensity losses that are inevitable in real world systems. Example losses include optical connector and cable losses and fiber bending losses. These losses “scale” or proportionally decrease the detected intensity values, and the scaling factor is not necessarily the same for different wavelengths.