Research On Optical Wavelength Detection Technology Based On Slow Light Interferometer

In the optical fiber sensor, the optical wavelength and frequency signals are less vulnerable to the environment disturbance. Therefore, it is prefer to modulate the measured parameters to wavelength or frequency signal to improve the system accuracy and for long-distance transmission. Then to obtain the measured parameters, it is needed to introduce the wavelength detection technology, which can demodulate the wavelength and frequency signals. So the wavelength detection technology with high sensitivity, high precision and good stability is significant for the optical fiber sensor. There are many wavelength detection methods, among which the Mach-Zehnder (M-Z) interferometer is the widely used technology for their simple structure, high sensitivity and good dynamic performance. However, there exist some problems in the traditional M-Z interferometer. Firstly, the output signals of interferometer are relative to the input intensities, so the shaking input light intensity will lead to inaccurate measurement. Secondly, the sensitivity at the edge of interference spectrum is almost zero, namely, there exists a measuring "dead zone". Moreover, the relationship curve between output signal and input wavelength (or frequency) is not linear, so the linear approximated method in practical wavelength measurement will introduce a system error for the measured results. Last but the most important problem is that only the length difference of two interferometer arms can be increased to improve the sensitivity of traditional M-Z interferometer. However, it will decrease the system stability in turn. Therefore, it is critical to find a way to effectively increase the sensitivity of interferometer.A high-sensitive M-Z interferometer was proposed in this paper by using slow light in a 1-dimentional grating device, and the system structure of interferometer was also improved. The main contents are as follows:(1) The design of M-Z interferometer wavelength detection system based on differential theory. Firstly, it was theoretically demonstrated that the influence of input light intensity on interferometer output could be eliminated by adopting differential technology. Then, an experimental system of M-Z interferometer based on differential theory was structured to verify this property, and an experimental wavelength resolution of 16 pm was realized.(2) The design of M-Z interferometer wavelength detection system based on differential and orthogonal theory. Firstly, it was theoretically demonstrated that the differential and orthogonal technology could not only eliminate the influence of input light intensity on interferometer output, but also could effectively improve the output spectrum properties of interferometer system, and could eliminate the measuring "dead zone" to further improve the linearity of output signal. Then, an experimental system of M-Z interferometer based on differential and orthogonal theory was structured to verify these properties o It is demonstrated that an experimental wavelength resolution of 8 pm and a simulated wavelength resolution of 0.14pm was realized.(3) The mathematical model establishment of slow light generation based on 1-dimentional grating device. By analyzing the geometry structure and optical properties of 1-dimentional grating device, the mathematical model of slow light generation was established, which could be used to simulate the transmission spectrum and group index spectrum of 1-dimentional grating device, and provide an effective method for analyzing the optical properties of 1-dimentional grating device.(4) The optimization of the structural parameters of 1-dimentional grating device, and the system design of M-Z interferometer based on the differential and orthogonal theory with slow light in 1-dimentional grating device. Combing the theoretical analysis and the above experimental results, the sensing properties of wavelength detection system were analyzed. By adopting the 1-dimentional grating device, the group index of 126 could be obtained, and the system resolution could be improved to 0.0011pm with the detection range of 0.20pm. Also, the group index of 10.7 could be obtained, and the system resolution could be improved to 0.013pm with the detection range of 6nm.The proposed Mach-Zehender interferometer in this paper could effectively improve the deficiencies of wavelength detection method based on traditional interferometer. The well improved wavelength detection resolution of the proposed system will further promote the application of interferometer in the optical fiber sensing field.