Polarimetric Heterodyning Fiber Grating Laser Sensors

With the rapid development of the information society, and the continuous improvement of the requirements for environmental monitoring, process control and infrastructure safety, people need higher performance sensor. Mordern sensor technology is developing to small size, network, multiple parameters and integrated. Fiber optical sensors as one of the most important sensors have been interested in and applied in military and cilvil fields, because of their advantage such as anti-electromagnetic interference, dexterity and multiplexing. Because of high reliability, strong stability, mature production technology and multiplexing ability by using of wavelength division multiplex, fiber grating sensors have been widely applied in the large structure for their integrity, security, load fatigue, injury severity and the state of real-time monitoring. Recently, fiber grating laser sensors as active optical fiber device have attracted much consideration, because of their narrow bandwidth and high signal to noise ratio, which permit more accuraty and resolution. Traditional fiber grating laser sensors is wavelength encoding. Like the principle of the fiber grating sensors, the equipments for wavelength demodulation are required, which is very expensive. Because of the small sensitivity of the wavelength encoding, the interferometric detection is required to read out the small wavelength shift. This greatly complicates the sensor multiplexing and increases the cost.In this paper, we propose and demonstrate a series of novel polarimetric heterodyning fbier grating laser sensors theoretically and experimentally. The multiplexing ability of the sensors based on frequency division multiplex is also investigated. The investigation has laid a solid foundation for the polarization heterodyne fiber laser sensor and its frequency division multiplexing sensor network to apply in future. The main research work and achievements are as follows:1. A novel fiber optic hydrophone based on polarization fiber grating laser is proposed and experimentally demonstrated. The principle of the proposed hydrophone is different in nature from the reported wavelength encoded fiber laser hydrophones. It uses dual polarization fiber grating laser as sensing element and converts acoustic signal into a change in the beat frequency between the two polarization modes from the laser. The proposed hydrophone has advantages of ease of interrogation, absolute frequency encoding, and capability to multiplex a number of sensors on a single fiber by use of frequency division multiplexing technique. The minimum detectable signal is0.03Pa at 400Hz. The frequency response exhibits a±7dB variation across the measurement bandwidth.2. Based on the structure of hydrophone, we propose and experimentally demonstrate a simultaneous measurement of temperature, hydrostatic pressure and acoustic signal sensor using a single DBR fiber laser. The acoustic wave induces a frequency modulation (FM) of the carrier in radio frequency (RF) range generated by the fiber laser and can be easily extracted by using the FM demodulation technique. The temperature can be determined by the laser wavelength. The hydrostatic pressure can be determined by monitoring the static shift of the carrier frequency and deducting the effect of the temperature.3. A hydrostatic pressure insensitive fiber optic hydrophone based on the integration of a dual polarization fiber grating laser and an elastic diaphragm is proposed and experimentally demonstrated. The diaphragm converts acoustic pressure into transversal force onto the fiber laser but shows no response to the hydrostatic pressure. Therefore, the beat frequency is sensitive to acoustic signal but insensitive to the hydrostatic pressure. It can be used in deep water.4. A novel simultaneous strain and temperature fiber optic sensor is proposed and experimentally demonstrated. The sensing head is formed by two concatenated ultra-short distributed Bragg reflector lasers that operate in single longitude mode with two polarization modes. The total length of the sensing head is only18mm. The two lasers generate two polarization mode beat notes in the radio-frequency range which show different frequency response to strain and temperature. Simultaneous strain and temperature measurement can be achieved by radio-frequency measurement.5. A strain-insensitive temperature sensor based on a dual polarization fiber grating laser is demonstrated. The measured temperature sensitivity is-78.46kHz/℃. In contrast, the sensor is almost insensitive to applied axial strain. The theoretical analysis is conducted, which give a reasonable explanation.6. To improve the frequency division multiplex ability of DBR fiber laser, we demonstrate a rotary-welding method to tune the beat frequency. By slicing the laser cavity into two sections and then aligning them with a rotated angle, the output beat frequency can be continuously tuned in a multi-octave frequency range as shown in the experiment from2.05GHz down to289MHz, as a result of the induced change in optical length for each polarization mode. Because the beat frequency can be continuously tuned in a multi-octave frequency range, this method can be used for a tunable RF signal generation.