Multiplexing of FFP Sensors Based on Microwave-Assisted Reconstruction

A method of multiplex fiber-optic interferometers for distributed sensing through microwave-assisted separation and reconstruction of optical interferograms in the spectrum domain is proposed [38]. Cascaded FFP sensors could be demodulated individually using this technique. The approach is schematically shown in Figure 5.22. The light from a broadband source is launched into a tunable optical filter and then intensity- modulated by a microwave signal whose modulation frequency can be scanned via computer control. The microwave-modulated light, where the optical signal is the carrier and the microwave is the envelope, is then sent into an optical fiber with cascaded reflective FFP sensors through a fiber-optic circulator. The reflection of each FFP sensor can be designed to be weak enough so that the light can be transmitted over many sensors and the multiple reflections within each sensor is negligible. The coherence length of the light, which is determined by the bandwidth of the tunable filter, is much larger than the OPD of the FFP sensors but much smaller than the distance between two adjacent

sensors. As a result, optical interference occurs within one sensor while the optical interference between sensors is avoided.

The optical detection is synchronized with the microwave modulation frequency so that the amplitude and the phase are obtained. After scanning the microwave frequency through the entire available range, the complex microwave reflection spectrum (with both amplitude and phase) is obtained. By applying a complex and inverse FFT to the microwave spectrum, a series of delta functions are obtained at discrete time positions. The discrete time domain signals are proportional to the optical interference signals of the cascaded FFP sensors at a particular optical wavelength (A,m) determined by the tunable filter, and also indicate the position of the sensors along the fiber.

By sweeping the optical wavelength and repeating the microwave measurement, the discrete optical interference signals (separated in the time domain) at different wavelengths are obtained. These data points can then be used to construct the optical interferograms of the cascaded FFP sensors. The concept of multiplexing of FFP sensors based on microwave-assisted reconstruction is like multi-wavelength time- division multiplexing (TDM) to some extent. Compared with TDM using laser pulses, it could achieve higher spatial resolution and obtain the full spectrum of sensors, and does not require all the sensors with the same OPD. The disadvantage of this approach is time consumption.

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