Study On The Fiber Oxygen Sensor Based On Locked-in Technology

The fiber optic sensing technology is a kind of new sensing technology developed in the middle of 20 century. With the development of fiber optic, spectroscopy, engineering and computer, there were increasing interests in the development of Fiber Optic Gas Sensor (FOGS) in 1980s. Nowadays the technique has been used in many fields such as chemical analysis, biology and medicine detection.In this thesis, based on the principle of luminescent quenching effect, we discuss the methods of detecting the luminescent. The so-called luminescent quenching is a kind of process that the luminescent material has chemical or physical reactions with certain substance. And then the luminescent process will change prominently. As a kind of applicable substance, Ru(phen)3cl2 was selected here. It has the advantage of good stability of luminescent, long life span and the strong quenching effect. Through the former experiment, we know that with the pulse light of blue LED. The sensor can emit the steady luminescent. So we can tell the density of O2 through the luminescent quenching time. But the optic fiber coupling effect in the light way reduces the intensity of the luminescent. So the PIN in the system for optic and electric changing just gets the faint luminescent. And what's more, the luminescent has the attenuation in exponential. It is difficult to detect the intensity of the luminescent in such condition. In time field, the signal, which is detected by PIN, is later than the inspired pulse light. And in frequent field, the phenomena can been seen as the locked phase lag in a certain frequency. This time, the method of "locked in" in detecting the faint luminescent is used. This technique is on the principle of correlation. It is useful for the periodical signal and makes the signal exclusively. With the effect of correlation between the signal and the noise, the signal becomes exclusive and much of the noise is wiped off.Based on the scheme of the locked-in technique, we make the design of locked-in amplifier. This electric-circuit consists of preamplifier, AC amplifier, multiplier and low-pass filter. Through such parts we can gain the result of the DC voltage that has the proportion to the locked lag phase. It is also called as phase sensing detection. Here we design the multiplier, correlator, to detect the lag phase. From the form data, we just know that the lag phase is about 1℃ at the preference frequency of 40KHz. So we made some kind of design. At first, we want to get the signals with little aberration. And we pay much attention on the frequency response. Thus the PIN selection is a problem. Because the PINwith big detect area has the bad frequency and the PIN with small detect area has the poor detect ability. We use the PIN with small detect area just for its good frequency response. And we just can get the faint current in this way. The signal must be amplified more than 50000. It is a big number. So the experiment is a hard task. Through we finish it, the result is not so well. The output range we get is less than 20mv.And the noise in the amplifier circuit we cannot wipe out. With the design of electric circuit, we found that it is hard to amplify the wave signal without aberration. There must be an integral part in the circuit. Through the experiment and theory deduce, we found the integral part useful to widen the lag phase. We can get about 5癈 of the lag phase. So we describe the signal with mathematics and prove that the integral part in the system do well in detecting signals. Also the experiment we have done reassure the opinion above.At the end of the paper, some ideas have been offered to promote the character of the detecting system.