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Interrogation and Multiplexing Techniques for ffp s ensors

Introduction

Generally, a typical FFP sensing system includes light source, sensor head, guiding optical fiber, sensing signal receiving, and demodulation [1], as shown in Figure 5.1. The interferometric fringe pattern of an FFP sensor is modulated by change of the optical phase difference (OPD) induced by the measurand, such as displacement, temperature, strain, pressure, etc. The crucial issue for interrogating the FFP sensor is to obtain the OPD variation from either the intensity or wavelength shift of the interferometric fringe. According to the way of obtaining the relative OPD variation and absolute OPD, the interrogating methods of FFP sensors could be classified into i ntensity i nterrogating, spectral interrogating, interferometric scanning, and white light interferometry (WLI).

Intensity Interrogating Methods and Instruments

Single-Wavelength Intensity Interrogating Method and Instrument

The schematic diagram of the intensity interrogating method is shown in Figure 5.2, for monitoring the OPD variation of an FFP sensor. Light from a laser diode (LD) or light-emitting diode (LED) passes through a circulator (or a fiber coupler) and is reflected by the FFP. After passing through the circulator again, the reflected light is converted by a photodiode to an electrical signal, which is processed electronically. Specially, it is required that the coherent length of the light source must be less than the OPD of the FFP sensor.

Typical configuration of an FFP sensor system

Figure 5.1 Typical configuration of an FFP sensor system.

Schematic diagram for monitoring the OPD variation of an FFP sensor

Figure 5.2 Schematic diagram for monitoring the OPD variation of an FFP sensor.

When the incident light intensity is constant, the cavity length of the sensor is a function of the wavelength and the output light intensity. Thus, in the vicinity of point E, the relationship between OPD variation and light intensity reflected is almost linear, as shown in Figure 5.3. Intensity interrogation is quite simple, but it is susceptible to fluctuation of the light intensity, vibration of the optical path, variation of the fiber transmission attenuation, and parameter drift of electrical circuits. An approach based on dual-wavelength carriers to eliminate the drift of the light source and optical/electrical links is proposed [2]. In addition, intensity interrogating puts forward very tight requirements on the consistency of sensors.

In the single-wavelength scheme, the linear dynamic range region of the OPD variation is limited to Relationship between OPD variation and light intensity reflected

Figure 5.3 Relationship between OPD variation and light intensity reflected.

A passive scheme that makes it possible to overcome this limitation has been implemented using two simultaneously monitored FFP sensors [3]. The interferometers are exposed to the same measurand and use the same light source, but they are fabricated so that their round-trip optical path lengths differ by an odd integral multiple of a quarter wavelength. By suitable processing of the two “quadrature- shifted” optical signals, it is possible to extract information on the measurand-induced phase shift while avoiding sensitivity nulls and ambiguity in the direction of phase change.

 
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