強度調變式光纖感測器及其關鍵元件之微型化研製

Abstract

Optical fiber has become the main transmission media in the telecommunication applications due to its excellent properties of low transmission loss, free from electromagnetic interference, small size, etc. Therefore, optical fibers are widely used in the local area network, cable TV, medical endoscope and industrial sensing system. This dissertation is focusing on the industrial sensing system development on the base of optical fibers. As we know, there are several modulation methodologies that can be adopted in an optical fiber sensor design. They include the intensity modulation, phase modulation, frequency modulation and polarization modulation. Among these, the intensity modulation type is the one we adopted for the sensor head design, which has the simplest architecture and lower cost to fit the industrial applications. There are three main topics in this dissertation. The first topic is concerned with the optical sensing system design. Based on the intensity modulation principle, we developed three types of sensor head for measuring the pressure, liquid level, and flow rate, separately. From the experimental results, we received very good linearity and accuracy for the sensor response which will be shown in the text in detail. The second topic is for improving the system stability under significant variation of light intensity due to the long-time operation and environmental perturbation. Since the intensity of the light source will be attenuated accompanying with the operation after a long time or varied along with the changes of environmental conditions. They will impact on the measuring accuracy. Therefore, we advised a solution by proposing a new structure of 4-fiber configuration, in which two collecting fibers are designed to receive the reflected light from the sensing element. In addition, a dummy fiber is placed between the transmitting fiber and the second collecting fiber to cause the light intensity difference received by the first and second collecting fibers at equivalent distance of d which is the distance between sensing element and sensor probe. By dividing the light intensities received by these two collecting fibers, we received a line with very good linearity in the range of d values from 300μm to 1000μm. Furthermore, we verified the performance of the new structure of 4-fiber configuration under great variation of light intensity by adjusting the LED driving current to be 30 mA and 20 mA to simulate the light variation. Test results show that the quotients of the intensities received by two collecting fibers at different d values are consistent between two conditions of LED driving current to be 30 mA and 20 mA. It proves the new solution proposed in this dissertation can improve the system stability under a greater variation of light intensity and keep the measuring accuracy successfully from the test results. The third topic is to propose an innovative process to fabricate the key components for both the optical sensing system and the optical network. By means of the two-step process combining the main techniques of twisted and parallel fusion processes in the fused biconical taper (FBT) technology, we successfully accomplish a hybrid device with the compact size of φ3.5 × 65 mm to accommodate a wavelength-division multiplexer and an optical splitter inside which is only 1/128 of the volume of a conventional package in a 100 × 80 × 10 mm module box. The innovative process to realize the miniaturization of the hybrid device can increase the flexibility during the installation due to the smaller package size. Besides, the hybrid device has the other excellent performance of ultralow polarization dependent loss (PDL) of less than 0.05 dB which ensures the quality of transmitted signals. Moreover, with passing the reliability tests based on the Telcordia GR-1209-CORE and GR-1221-CORE, we have successfully demonstrated that the hybrid device we designed can meet the stringent requirements for various applications under adverse environments.