Mechanism And Characteristics Of Ultra-narrow-linewidth Brillouin/erbium Fiber Ring Laser

Ulta-narrow-linewidth lasers(~k Hz)have found important applications in many fields,such as interferometric fiber sensors,long-distance coherent optical fiber communications,high-accuracy optical measuring,and high-resolution LIDAR.The linewidth of the laser sourc affects the performance of the system.Especially,the ultra-narrow-linewidth laser source is in demand of both stable fast tuning and central frequency in the interferometric fiber sensors based on phase generated carrier technique.The generally used ultra-narrow-linewidth solid laser,semiconductor laser,and erbium-doped fiber laser each have some limits and are unable to present the above all characteristics.Ultra-narrow-linewidth fiber lasers based on stimulated Brillouin scattering(SBS)in fibers have been proposed and attracted much research attenetion,with the development of fiber nonlinear effects.The Brillouin fiber laser(BFL)has been still insufficient for applications for the past fourty years,for it needs critical pump-coupled resonator or watt-scale pump powers.In this dissertation,it is initially proposed to use a length of several meters erbium-doped fiber(EDF)as both the Brillouin and linear gain media,so that a single-frequency compact Brillouin/erbium fiber laser(BEFL)of low threshold(~ 30 m W),low phase noise(-125 d B/Hz1/2 at 1 k Hz with 1 m optical phase difference)is realized.Meanwhile,this BEFL presents ultra-narrow linewidth(~ 50 Hz),stable fast tuning,and stable central frequency,showing great importantance in many applications such as interferometric fiber sensors,coherent communications,and microwave photonics.The main results and innovations are as following::1?The compact BEFL with less than 10 m cavity length is realized based on the method of using a EDF providing both the Brillouin and linear gain.The typical bandwith of fiber Brillouin gain spectrum is 20 MHz,demanding the cavity length shorter than 10 m for stable single-mode operation of the laser.High-nonlinearity fibers were used for shortening cavity length.This method,however,causes high pump threshold and low output power for the laser.The compact BEFL based on normal EDF has short cavity length and single mode.Meanwhile,it is much easier to be realized.2?The key physical effect of the compact BEFL,i.e.,the stimulated Brillouin scattering(SBS)is studied.Experimental results show that the Brillouin frequency shift is 11.13 GHz,and the bandwidth of the Brillouin gain spectrum is 50 MHz for the EDF.The Brillouin pump threshold of the EDF is derived,showing that the linear gain of the EDF can lower the Brillouin pump threshold effectively.Especially when the EDF is in a resonator,the Brillouin pump threshold is reduced to several miliwatts.The study on characteristics of SBS of the EDF is the base for the research on the compact BEFL.It also extend the content of the study about fiber SBS.3 ? The characteristics of the BEFL power is studied.Both the theoretical experimental results demonstrate the low threshold and high output power of the BEFL.The BEFLs using 4 m,1.5 m,and 45 cm EDF are constructed,respectively.Brillouin pump of 1 m W power is sufficient for them,and the 980 nm pump threshold is only 30 m W.Compared with the short-cavity BEFL using high-nonlinearity fibers,the BEFL in the dissertation presents the advantages of low threshold and high power.4 ? The characteristics of the BEFL ultra-narrow linewidth is studied.The narrowband Brillouin gain spectrum is the natural linewdith-narrowing mechanism for the BEFL.The BEFL can transfer the Brillouin pump with broader linewidth to the ultra-narrow-linewidth Brillouin Stokes emission.It is experimentally demonstrated that the compact BEFL transfer the 1.12 MHz Brillouin pump to 1 k Hz Brillouin Stokes.The Lorentzian linewidthof the BEFL is indicated to be only about 50 Hz using the measureing method based on Voigt profile fitting.The phase noise of the compact BEFL is demonstrated to be-125 d B/Hz1/2 at 1k Hz(The optical path difference of the fiber ibterferometer is 1m)?The ultra-narrow linewidth of the BEFL and the capacity of narrowing laser linewdith are meaningful for the applications in coherent optical systems.5?The characteristics of fast tuning of the BEFL is studied.The fast-tuning BEFL is realized based on the stretching effect of the piezoelectricity crystal ring.It is experimentally demonstrated that the fast-tuning BEFL presents about 60 MHz frequency tuning range,maximum 48 k Hz tuning rate,-124 d B/Hz1/2 phase noise at 1 k Hz(the optical path difference of the fiber interferometer is normalized to 1 m),and high fast-tuning stability.The fast-tuning BEFL is more stable than the fast-tuning laser diodes,and its central frequency is more stable that that of the erbium-doped fiber lasers based on saturable absorbers.The fast-tuning BEFL permits the enhancement of stability and accuracy of the sensing systems.6?The multi-wavelength BEFL with over 50 GHz frequency space is studied.Such kind of BEFL is realized by injecting different Brillouin pump with over 50 GHz frequency space in the cavity.A dual-wavelength pre-pump BEFL is realized and its frequencys space is tunable.The multi-wavelength BEFL with over 50 GHz frequency space presents wide applications in wavelength-division-multiplexing systems.