| Due to the coherent length and phase noise are directly decided by the laser linewidth,ultra-narrow linewidth laser has become an indispensable light source in many civilian/military fields,to name but a few,coherent optical communication,distributed fiber sensing system,laser LIDAR,and laser Interferometer gravitational-wave observatory(LIGO).And with the rapid development of microwave photonics and other new fields,their requirements for laser source’s linewidth is increasingly demanding.Therefore,it is desirable to obtain an ultra-narrow linewidth laser operating at a single longitudinal mode(SLM)state under normal conditions.There are various types of laser light sources,however,fiber laser and semiconductor laser are the predominant types of lasers in today’s industrial and academic applications.Up to now,whether it is a fiber laser or a semiconductor laser,the frequency stabilization technology based on an external servo-electrical feedback and an optical feedback technology are common method employed to achieve narrow linewidth laser.However,the electrical feedback acting as a means of laser frequency stabilization requires external detection and control devices with high precision and sensitivity.It also requires the accurate control for the operating environment of the reference cavity which ultimately leads to a high cost.Moreover,the optical injection feedback is a power feedback from an external cavity with a fixed length,which is arduous to realize the deep compression of laser linewidth on the basis of ensuring the SLM operation.Therefore,it remains a challenge to further realize a deep compression of laser linewidth even upon the attainment of an SLM state for the field of scientific and industrial applications.This paper starts from the theoretical analysis with an exclusive focus on the principle of deep compression of laser linewidth,by leveraging fiber laser,and semiconductor laser as typical examples in the research.It further proposes a variety of ultra-narrow linewidth laser schemes,and realizes laser output with hundred order of linewidth.The main contents of this thesis are as followings:(1)In view of the limitations of traditional narrow linewidth lasers,the principle of deep laser linewidth compression is further studiedbased on the existing work of our research team.Firstly,an external cavity laser model assisted by Rayleigh scattering feedback is proposed.Starting from the characteristic parameters such as intensity I(λ)and phase θ(λ)of reflection interference spectrum,the relationship between them and wavelength λ is deduced,which reveals the matching characteristics of the feedback signal from the effective scattering feedback planes to the main cavity.And the principle of realizing the deep compression of laser linewidth is analyzed based on this condition.Furthermore,taking Rayleigh scattering as a natural feedback mechanism,the frequency spectrum evolution characteristics of Rayleigh scattering in one-dimensional waveguide is investigated based on the quantum characteristics of light scattering.The frequency spectrum evolution model of Rayleigh scattering source during the continuous scattering process is established by considering system dissipation,which further reveals that the Rayleigh scatteringcan provide a weak feedback signal with a continuous narrowing in frequency domain for the laser gain during the oscillation process.Finally,the formula of laser linewidth corresponding to the laser oscillation is derived by analyzing the influence of the feedback light field on the thermodynamic potential,and the role of Rayleigh scattering in laser linewidth compression is revealed by the corresponding theoretical simulation.The study of this work provides a powerful theoretical reference for the deep compression of laser linewidth.(2)Based on the deep compression principle of laser linewidth,the experimental studies of deep linewidth compression of fiber lasers are conducted.Firstly,a fiber ring laser with narrow linewidth has been proposed,and the linewidth compression law assisted by Rayleigh scattering is investigated in a 1 μm laser system.A laser operating at 1064.4 nm with a 1.35 k Hz linewidth and a SMSR of 50 d B is obtained.Secondly,the deep linewidth compression law of a linear cavity laser is investigated,and an ultra-narrow linewidth linear cavity laser assisted by Rayleigh backscattering(RBS)is proposed.The experimental results under different length of the scattering fiber reveal that increasing the length of scattering fiber is the primary method to accumulate RBS signal effectively,provided the Rayleigh backscattering coefficient(RSC)is invariant.When RBS fiber length is increased to 5 km,an ultra-narrow linewidth laser with an output linewidth less than 210 Hz,with an SMSR greater than 60 d B in absence of any other nonlinear phenomena is obtained.The results of those work confirm the deep compression affection of Rayleigh scattering for laser linewidth and the wavelength independence.They also provides a suitable method for the realization of other gain-band and multi-wavelength laser linewidth compression at the same time.(3)Based on the deep compression principle of laser linewidth,the experimental studies of deep linewidth compression of semiconductor lasers are conducted.Firstly,an ultra-narrow linewidth laser based on a through coupling structure is proposed.The evolution law of laser linewidth under different scattering fiber length is investigated using the single mode fiber(SMF)-28 e as scattering fiber.An ultra-narrow linewidth laser with SMSR greater than 55 d B and output linewidth of approximately 235 Hz is realized when the scattering fiber length is 700 m.Furthermore,an ultra-narrow linewidth laser based on a beam splitting coupling structure is proposed.Compared with the straight-through coupling structure,the injected power into the RBS fiber could be controlled using a 30/70 coupler.An ultra-narrow linewidth semiconductor laser with an output linewith of 175 Hz and a SMSR of 60 d B is obtained with the assistance of a 5km SMF-28 e.Based on this,an enhancement characteristics of the Rayleigh scattering in an Erbium-doped fiber(EDF)-980-HP is investigated by using the OFDR technology.It is first revealed that increasing the RSC of the scattering fiber is crucial to shortening the scattering fiber’s length by a comparative experiment between SMF-28 e and EDF-980-HP.The results of those work provide an effective method to realize the ultra-narrow linewidth output of semiconductor lasers and have a powerful reference significance for the miniaturization and the portable development of ultra-narrow linewidth lasers.In this thesis,the contribution of Rayleigh scattering in deep linewidth compression of laser linewidth has been investigated from the deep compression principle to the experimental system.Based on the principle of deep linewidth compression,we have conducted the corresponding experimental investigations about fiber lasers and semiconductor lasers,and proposed a variety of ultra-narrow linewidth laser configurations.The wavelength independence of Rayleigh scattering in laser linewidth deep compression is proved by investigated the compression law in different lasing wavebands,which has a very important reference significance for realizing a deep compression of other gain bands and multi wavelength laser.In addition,the proposed semiconductor laser configurations not only provide an effective method to obtain ultra-narrow linewidth semiconductor source,also have an powerful scientific reference for realizing the miniaturization and portable development of ultra-narrow linewidth laser by a comparative experiment of between the fibers with different RSC. |
