Fiber-optic Sensors Based On Metal Nano-wire Grid

In last three decades, fiber-optic sensing technology has been intensively studied. Divided by different theories and structures, they can be applied for measuring different physical quantities, like temperature, strain, gas concentration, electric current, etc. In many fields, the fiber-optic sensing technology is starting to replace traditional measuring methods, as the fiber-optic sensing technology has lots of particular advantages, such as lightweight, compact size, immunity to electromagnetic interference, being compatible with modern fiber-optic network and so on. In this thesis, we design a kind of metal nano-wire grid (NWG) on tip of an optical fiber, which has a function of splitting light beams of different polarization states. As a result, it can be regarded as an integrated counterpart of the Wollaston prism. Based on this innovation, we build up two fiber-optic sensors for sensing electric current and pressure, respectively. The thesis can be divided into following three aspects in detail:(1) The NWG we design is gold wire grating fabricated on a fiber-tip, whose period is far smaller than the wavelength of the incident light in the fiber. The NWG has an interesting characteristic of reflecting TE mode part of incident light while transmit the TM part. Both of the reflected and transited light can be collected and detected simultaneously, so the NWG can be regarded as an integrated counterpart of the Wollaston prism. We fabricate the NWG with the help of Focused Ion Beam technology. The period and duty cycle of the NWG are200nm and0.5respectively. The sample has a16.3dB reflection contrast and a4.8dB transmission contrast.We package the sample within a tiny glass-tube to achieve a compact in-line fiber polarizer.(2) Based on the fiber-tip NWG, we build up an all-fiber optical current sensing system. The sensing system uses Faraday Effect and employs the optical fiber itself as the sensing head. In order to overcome the disadvantages of existing birefringence and low Verdet coefficient of an optical fiber, we use a Faraday rotator mirror to achieve a reflective structure. With the help of the NWG, the sensor becomes an in-line optical heterodyne system. Compared with the sensors using Wollaston prism, it is more compacted and of much optical stability. The experiment result proves that the system is able to withstand light power variations showing good sensing stability.(3) We also design a fiber-optic pressure sensor by using the fiber-tip NWG. It is based on the photo-elastic effect, and it is an all-fiber system without any bulk optic element, too. The pressure sensitivity is measured of0.237rad/N. Similar to the current sensor we design, the pressure sensor also has advantages of optical stability, heterodyne detecting and so on. In additional, besides the pressure sensing, the system can apply onto vibration sensing. According to our experiments, frequency response for vibration sensing from20Hz to4kHz is characterized showing nice linearity.