Research On Suppresion Technique Of Intensity Noise In Three-level Fiber Laser

With the development of optical communication and optical detection,high quality laser beam is urgently demanded.The output wavelength of three-level fiber laser is 1.55 ?m,which is located in the third communication window.It's eyesafe and can be better compatible with existing optical communication devices.So it has wide development space in the human eyesafe laser ranging,homodyne coherent optical communication and other fields.This thesis studies on suppressing noise of erbium-doped fiber laser with the photoelectric feedback loop method.The main research contents include:(1)This thesis designed a short cavity fiber laser based on high concentration erbium-doped fiber.The length of doped fiber is 2.2 cm.The fiber laser output power is 0.87 mW,and output wavelength is 1540 ± 0.1 nm which can change with the temperature.(2)We analysed constitution of laser noise,which consist of the fluctuation of the pump,interference,mechanical vibration,cavity loss and spontaneous radiation.The interference caused by fluctuation of the pump and cavity loss is far more than other factors.Through the rate equation,we obtain equations of relaxation oscillation noise.(3)A temperature control system was designed.We use the temperature controller and semiconductor refrigerator to control the temperature.Through the PI feedback control,we can effectively reduce the temperature fluctuation,and avoid noise caused by temperature fluctuations.According to the experiment test,temperature fluctuation is controlled in 0.05 ?.(4)We amplify the seed laser beam by MOPA structure.Then we obtain the relationship of gain with the change of the pump power and with the change of signal light power.The output power of 24 cm and 72 cm length EDFA were tested and we analyse its saturable output characteristic with experiment data.(5)The feedback control circuit is designed.Feedback control circuit consists of noise detection,signal amplification and the driving circuit.We analysis the characteristics of AC circuit with software Tina.The circuit open-loop transfer function of the circuit is given,and the whole circuit has stable amplitude and phase in 1 kHz to 10 MHz.(6)We show the bode diagram of direct modulation,fiber amplifier and feedback circuits respectively and test the single frequency noise suppression effect.Single frequency noise is suppressed by 20 d B and the laser noise is suppressed by 10 dB.