In this paper, an optical fiber length measurement system based on phase-modulated pulse time of flight method is proposed. The fiber length is calculated by measuring the flight time of the phase-modulated pulse in the fiber under test. The measurement system is based on the interference structure of Mach–Zehnder hybrid Sagnac to obtain stable interference effects. To improve the signal-to-noise ratio of the system, the phase-generated carrier technique is utilized to effectively eliminate the parasitic Rayleigh scattering noise along the optical fiber. Finally, a balance of long measurement distance and high measurement precision is achieved in the system, and a measurement resolution of 0.2 m is realized within the measurement range of 100 km.

Owing to increasingly severe environmental pollution, food safety and other problems, higher and higher requirements for the detecting technique of poisonous and harmful biochemical molecules have been put forward. The conventional biochemical detector has the disadvantages of large size, high cost and inability to realize far-end and in-situ detection functions. Based on the requirements of the biochemical molecular detection technology for high sensitivity, miniaturization, far-end detection, insitu detection, real-time analysis and the like, a detection method using a fiber surface-enhanced Raman scattering (SERS) probe to carry out Raman signal detection has been put forward in recent years. The detection method not only realizes far-end and insitu detection functions, but also has a relatively high sensitivity. In this paper, a taper and cylinder combination type fiber probe is made by adopting a simple tube corrosion method, Under the situation of fixed temperature, cone-cylinder combined fiber probes with different diameters are obtained by controlling the corrosion time, and silver nanoparticles are bound to the surface of a silanized silicon dioxide fiber probe through electrostatic forces. Then, the sizes and morphologies of silver nanoparticles on the surface of the fiber probe are observed under a scanning electron microscope. Besides, the detection limit of a rhodamine 6G (R6G) solution is used to manifest both the activity and the sensitivity of the fiber probe, and the self-assembly time of the silver nanoparticles are further optimized to be 30 min and the diameter of the fiber probe to be 62 upm. When the concentration of a silver sol solution is constant, a high-sensitivity fiber SERS probe can be prepared. Through far-end detection, the detection limit of the R6G can reach 10-14 mol/L, and the enhancement factor is 1.36104. This work can serve as an experimental basis for a novel fiber surface-enhanced Raman scattering sensor in such aspects as high sensitivity and low cost. The studies of this paper are expected to provide an appropriate detection technique for rapid quantitative detection of biochemical molecules, and further provide a reference for various application fields of environmental monitoring and food safety analysis in future in terms of realizing rapid and accurate in-situ detection. Therefore, the fiber SERS probe has large application foreground in molecular detection.