Study On Noise Mechanisms And Its Suppression Technique Of Phosphate Single-frequency Fiber Laser

Over the past two decades, single-frequency fiber lasers have undergone spectacular advances for its unmatched roles in a variety of application fields such as coherent optical communication, interferometric sensing, coherent beam combing, etc. At present, researchers still dedicate to improve the performances of single-frequency fiber lasers, to continuously improve its level of application and develop new application fields. As the important indicator of single-frequency fiber laser, noise properties have been recognized as the key factor that limiting its application level. As phosphate single-frequency fiber laser is an important outcome of the current single-frequency laser technology, it is of great significance to study its noise properties and related suppression technology. This paper aimed to deeply study the noise mechanisms of phosphate single-frequency fiber laser and explore new noise suppression technologies, and made the following achievement:Analyzed the intensity noise mechanism of high-gain phosphate single-frequency fiber laser, and experimentally verified the amplified spontaneous emission(ASE) noise component in intensity noise. Built the theoretical model of pump induced frequency noise(self-heating noise) in high-gain phosphate single-frequency fiber laser, and obtained the corresponding noise expression. In addition, analyzed the affecting mechanism of ASE on frequency noise under low power condition. By constructing phosphate single-frequency fiber laser system, the self-heating noise as well as the ASE noise were verified experimentally. Finally, analyzed the coupling mechanism between intensity noise and frequency noise in high-gain single-frequency fiber laser, and further verified by designing corresponding experiment.Based on the relationship between frequency noise and linewidth of single-frequency laser, we exploited the loss-compensated recirculating delayed self-heterodyne(LC-RDSH) method to study the frequency noise and linewidth of high gain phosphate single-frequency fiber laser, and further verified the self-heating noise and ASE noise. In addition, we also used this method to measure the linewidth of single-frequency laser at 1.0 μm, and achieved a measurement resolution of 3 k Hz with 6 km of delay fiber.We utilized a liquid crystal device to suppress the low-frequency intensity noise of single-frequency fiber lasers, and achieved over 10 d B of reduction in frequencies from 0.25 k Hz to 1 k Hz; moreover, the relative intensity noise(RIN) at frequency of 1 k Hz is below-140 d B/Hz. By exploiting the nonlinear amplification characteristics of saturated semiconductor optical amplifier(SOA), we realized about 20 d B of suppression on intensity noise of single-frequency laser at frequency band of 0.2-5 MHz. Additionally, based on the passive intensity noise suppression effect in the nonlinear amplification process of SOA, we proposed the intensity noise suppression scheme of cascading two SOAs, and achieved noise suppression band of over 50 MHz and about 30 d B of reduction to laser relaxation oscillation peak.Designed and built forward and backward self-injection locking structure for single-frequency fiber laser, and obtained considerable suppression for frequency noise and linewidth. By exploiting recursive superposition characteristics of intensity noise in self-injection locked laser system, and the recursive noise reduction effect of SOA, we designed and built SOA incorporated self-injection locking system for single-frequency fiber laser, and the frequency and intensity noise of the laser are remarkably suppressed in a significantly wide frequency range. A maximum reduction of about 25 d B to the laser frequency noise is achieved, while the laser linewidth is suppressed from 3.5 k Hz to about 700 Hz. After suppression, the RIN is within 5 d B of the shot noise limit at frequencies from 1.5 to 3 MHz and the frequency range of the suppression reaches about 30 MHz. The relaxation oscillation peak is observed to shift from about 1 MHz to 20 k Hz, and is reduced by about 35 d B from-90 d B/Hz to-125 d B/Hz.