Research On Low Coherence Fiber Differentiating Interferometer And Its Demodulation Schemes

In this paper, a low coherence fiber differentiating interferometer and its passive demodulation schemes are presented. The interferometer which adopts a broadband light resource is a hybrid configuration of Mach-Zehnder and Sagnac interferometer. By a proper demodulate algorithm, this fiber differentiating interferometer could be used in phase measurement. Furthermore, the absolute amplitudes are obtained by a triple detection fiber differentiating interferometer which is based on triple detection passive demodulation. On the basis of differential interferometer, a dynamic range adjustable interferometer is presented by the analyzing of phase compression theory.Low coherence fiber differentiating interferometer is based on the temporal coherence principle of interferometer. In this interferometer, the signal beam and reference beam travel along the same path but in opposite directions. Phase compression theory leads to the actual phase shift being compressed much more times, which extends the range of dynamic measurement.Signal demodulation plays an important role in fiber optic interferometer sensor technology. Low coherence fiber differentiating interferometer has some advantages such as with large dynamic range, without phase biasing and void low frequency environmental disturbance naturally. It is suitable for passive demodulation. A passive demodulation scheme employs a differential-and-crossmultiply (DCM) method to obtain the vibration signal. For resolving the two key problems of direct current component elimination and absolute amplitude measurement, we present a triple detection passive demodulation scheme which is based on three detectors.Phase compression factor (PCF) can change the phase compression rate of actual phase difference. The value of PCF is related to signal frequency and delay fiber length. Low-frequency signal leads to a bigger PCF, and the actual phase shift could be compressed much more times. A low coherence fiber differential interferometer with adjustable measurement range is presented in this paper. The measurement range of differential interferometer is related to the PCF, which can be adjusted by an optical switch.Low coherence optical fiber differential interferometer in the remote vibration measurement, refractive index measurements, current measurements and many other fields has its potential application value.The followings are the major study tasks md achievements in the paper.1. We present a low coherence fiber differentiating interferometer and its passive demodulation schemes. It has some advantages such as with large dynamic range, without phase biasing and void low frequency environmental disturbance naturally.2. The low coherence fiber differentiating interferometer with three detectors can realize the real-time measurement of absolute phase amplitude. The experimental results demonstrate that the phase demodulation resolution is 6×10-5rad and the maximum measuring amplitude is up to 90rad.3. A low coherence fiber differential interferometer with adjustable measurement range is presented in this paper. The measurement range of differential interferometer is related to the delay fiber length, which can be adjusted by an optical switch. The experimental results demonstrate that the phase demodulation resolution is 3.37 ×10-5 rad and the maximum measuring amplitude is 419rad with signal frequency of 1 kHz.