tailieunhanh - Overview of Fiber Optic Sensors phần 3

Tham khảo tài liệu 'overview of fiber optic sensors phần 3', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Figure 32 shows the basic elements of a Mach-Zehnder interferometer which are a light source coupler module a transducer and a homodyne demodulator. The light source module usually consists of a long coherence length isolated laser diode a beam splitter to produce two light beams and a means of coupling the beams to the two legs of the transducer. The transducer is configured to sense an environmental effect by isolating one light beam from the environmental effect and using the action of the environmental effect on the transducer is to induce an optical path length difference between the two light beams. Typically a homodyne demodulator is used to detect the difference in optical path length various heterodyne schemes have also been used . 43 . Transducer Light Source Coupler Module f Homodyne Demodulator Figure 32. The basic elements of the fiber optic Mach-Zehnder interferometer are a light source module to split a light beam into two paths a transducer used to cause an environmentally dependent differential optical path length between the two light beams and a demodulator that measures the resulting path length difference between the two light beams. One of the basic issues with the Mach-Zehnder interferometer is that the sensitivity will vary as a function of the relative phase of the light beams in the two legs of the interferometer as shown in Figure 33. One way to solve the signal fading problem is to introduce a piezoelectric fiber stretcher into one of the legs and adjust the relative path length of the two legs for optimum sensitivity. Another approach has the same quadrature solution as the grating based fiber sensors discussed earlier. Figure 33. In the absence of compensating demodulation methods the sensitivity of the Mach-Zehnder varies with the relative phase between the two light beams. It falls to low levels when the light beams are completely in or out of phase. Figure 34 illustrates a homodyne demodulator. The demodulator consists of two .