Study On The System Of Quasi Distributed Optical Fiber Temperature Annunciator Based On Rayleigh Backscattering

The QDOFTA (quasi distributed optical fiber temperature annunciator) is a completely new annunciator technology, developing rapidly in order to meet the urgent need of modern engineering technology. It can real-time monitor the temperature in such important establishment as mine,oil field,tunnel and intelligent building to make them run safely. So the stability of temperature annunciator system is crucial for its application, and it is basic assurance for this technology to be used widely too. We have researched the stability of such temperature annunciator system as is based on the Rayleigh backscattering in optical fiber. Concretely analyzing the main factors that affect the annunciator system's stability, we have improved the stability to large extent by taking the following measures:(1) Enhancing the precision of DAC and ADC. It is very important to control APD and LD's reverse voltage by DAC and sample their reverse voltage by ADC while the QDOFTA system is running. And it is also important to sample APD's temperature by ADC too. System has to real-time control APD's reverse voltage according to the variation of APD's temperature so as to compensate APD's thermo-drift to let it work with a nearly invariable gain. And system needs to control LD's reverse voltage to let it work safely too. All of these must been realized on the condition that the precision of DAC and ADC has been greatly improved.(2) Compensating APD's thermo-drift with reverse voltage It can be proved in theory that APD's gain is the function of its reverse voltage and temperature, as can be said that the reverse voltage, together with temperature, determines APD's gain while it is working normally. Moreover there is a linear relation between APD's reverse voltage and temperature if its gain is basically kept invariable. So APD's reverse voltage can be controlled to vary with temperature according to a determinate rule so as to maintain APD to work normally on condition that its gain is basically kept constant. This is the theory of compensating for APD's thermo-drift with reverse voltage.(3) Designing the working circuit for LD (laser diode)We should design the circuit of ATC (automatic temperature control) and and APC(automatic power control) for LD to let it launch stable optical wave and output stable power too. And we also need design the driving circuit for LD to make itwork with high speed. (4) Compiling and optimizing the system soft.To process the data acquired by system is the most important tache for improving the annunciator's monitoring veracity. So the data processing should eliminate such error as is needless to avoid distorting the performance parameter of system. So it is obvious that the system software's running stably is the integrative behave of the annunciator system's stability.(5) Designing the annunciator's temperature detector The annunciator's sensitivity is depended on the design of temperature detector to large extent. While the system is running, the optical fiber, which is nipped in the detector, will bebended to make its attenuation coefficient vary if the temperature rise, and then system gives a temperature alarm by having measured that the attenuation coefficient of the optical fiber has varied suddenly and overstepped the determinant value. Moreover, to what extent the optical fiber should be bended when the system gives an alarm has to be determined by doing a lot of experiment. After having completed above work, we can debug the whole system by optimizing the system soft. This is because some shortcoming of the hardware system can be remedied by the software's assistant function. By debugging we can improve the annunciator's performance to large extent so as to advance the system's stability of QDOFTA.