Fiber Laser Sensor Base On Beat Frequency Demodulation Method

Fiber laser sensors have attracted more and more attention owing to their high signal-to-noise ratio and narrow linewidth, which determine that sensors have an inherent high resolution. These advantages promote the application of the fiber laser. Such as hydrophone, accelerometer, acoustic microphone, and strain sensor. In this paper, we review several main demodulations method of fiber laser. The beat frequency demodulation method is one of the most popular technologies. This approach offers a simpler optical and electric signal processing solution than conventional interferometric scheme by measuring the beat frequency using a radio frequency spectrum analyzer. A number of polarimentric laser sensors, base on the demodulation method, were used in various applications including hydrostatic pressure sensor, load sensor, ultrasonic hydrophone, etc. Nevertheless, in fact, the single-mode operation of two orthogonal polarizations laser is a little difficult to establish and the repetition may be not stable. In addition, great care is taken to ensure that the beat signal of two orthogonal polarization modes can be easily monitored by mixing two polarization modes with a polarizer. To solve these problems, we proposed several fiber laser sensors and demonstrated experimentally.In the second section, a novel dual-wavelength fiber laser sensor is proposed and demonstrated experimentally. A beat frequency sensing signal, which is generated by the coherently mixing output of a dual-wavelength laser in a high frequency photodetector, has been obtained and demodulated by a radio-frequency spectrum analyzer. By employing a LiNbO3 modulator, high-frequency sensing signal can be tuned arbitrarily to tens or hundreds of megahertz without distortion and thus a low-frequency frequency spectrum analyzer can be used. When a strain is applied on the laser sensor, the detected beating signals will be shifted with a strain sensitivity of about -8.1 KHz/με. The minimal detectable strain of the sensor system is about 5με.In the third section, a multi-longitudinal mode fiber laser sensor is proposed. Firstly, A linear cavity laser is formed by two fiber Bragg gratings and a piece of erbium-doped fiber. When the strain is applied on the sensor, the laser cavity is stretched, which leads to a change of round-trip frequency. Thus the strain can be obtained by measuring the beat frequency of the resonant cavity. Experimental results show that the sensor has a sensitivity of -1.1 KHz/με@1413MHz and the root-mean-square deviation of 3.6με. However, it is difficult to fabricate two perfectly matched FBGs to form a very stable laser cavity. Some tiny mismatches of wavelength and phase of the two FBG reflectors will bring more noise, leading to the significant degeneration of SNR. And then, a multi-longitudinal modes fiber ring laser sensor is proposed. The ring cavity of the laser is formed by a 3-dB coupler, a section of erbium-doped fiber and one fiber Bragg grating. Photonic generation of beat signals and strain measurement theory are discussed in detail. A selection way of the optimal beat signals for strain measurement is identified by experimental research. The root-mean-square deviation of the strain and the response of beat frequency to the strain are 2.7μεand -1.5 kHz/μεat 1993 MHz, respectively. The proposed sensor scheme offers a cost-effective and high-stability device for strain measurement.In the fourth section, a simple and low cost dynamic frequency-demodulation scheme is proposed base on phase-shift discriminator. The original sensing information can be extracted by this technology. And then dynamic signal is obtained by a 16-bit analog to digital converter. It is demonstrated that the vibration from 60 Hz to 20 kHz can be measured successfully. This technology offer a very low value frequency shift detection scheme and is sufficient for vibration studies in many smart structure application.At the last section, a simple hybrid wire-wireless fiber laser sensor is proposed. In the sensor, an improved multi-longitudinal modes fiber laser cavity is set up by only a fiber Bragg grating, a section of erbium-doped fiber and a broadband reflector. And then the beat signal including the sensor information can access the wireless network through the wireless transmission. At last, a frequency spectrum analyzer is used to demodulate the sensing information. With this method, the long distance real-time monitor of the fiber sensor can be realized. The proposed technique offers a simple and cheap way for sensing information of the fiber sensor to access wireless sensor network. An experiment was implemented to measure the strain and the corresponding root mean square deviation is about 5.7με@916 MHz and 3.8με@1713 MHz after wireless transmission.