Research On Improvement Of External Cavity And Linewidth Measurement Of External Cavity Tunable Laser

With its excellent perform,external cavity tunable lasers(ECTL)have been widely used in coherent optical communication,optical fiber sensing and other fields.However,with the development of technology,its also puts forward higher performance requirements.The external cavity of tunable laser plays a decisive role in the performance of output laser.In order to obtain a single-mode output,narrower linewidth,larger continuous tunable range,higher output power and more stable light source,the external cavity of the laser needs to be continuously improved.The linewidth level of laser is also related to the performance of application system,especially in the field of optical communication and optical measurement.The size of linewidth affects the quality and level of communication and plays a decisive role in the distance,accuracy and sensitivity of optical measurement.Therefore,it is of great significance to study the accurate measurement and characterization of laser linewidth.In this paper,the improvement of the narrow linewidth external cavity tunable laser and the linewidth measurement are systematically studied.Firstly,we improve the external cavity filter of the external cavity tunable laser and propose an external cavity filter combination based on the parallel periscope and analyze its principle and theoretical derivation.The theoretical analysis model is given.The measurement methods of laser linewidth are summarized and the delayed self heterodyne method is deeply exploredand.A linewidth measurement scheme based on FRM(Faraday rotating mirror)is presented,and the principle analysis and experimental exploration are carried out.The specific research contents and achievements of this paper are as follows:Firstly,the working principle of the external cavity tunable laser is studied and the key components of the tunable laser are analyzed and introduced.A novel external cavity filter consists of a parallel periscope,a etalon and a narrow-band filter for tunable laser is proposed and studied..In this paper,the principle of mode selection is studied and analyzed and the wavelength tuning function is verified.The analysis shows that this design has no dependence on the rotation axis point when tuning the wavelength,which can improve the stability of the laser and prevent the occurrence of mode hopping and has practical engineering application value;In addition,the new filter assembly can further narrow the linewidth by realizing dual filter mode locking through three cavity coupling.the relationship between the parameters of the external cavity(periscope incidence angle,distance between parallel periscopes,distance between cavity mirrors,incidence angle of the etalon)under the condition of ideal tuning without mode hopping is derived.According to the theoretical analysis and reasoning of the parameters of the new external cavity design,we have designed,simulated and optimized the optical path of the laser with an output wavelength of 650nm based on the new external cavity design and the tunable range without mode hopping is analyzed.Secondly,the theory of laser linewidth is introduced and the working principle of linewidth measurement method is analyzed.According to the different principles,the linewidth measurement methods are divided into two categories:the linewidth measurement method based on signal power spectrum and the linewidth measurement method based on phase noise spectrum.The advantages and disadvantages of these methods and the corresponding detection range are combed and compared in the form of tables.The principle analysis and simulation of the commonly used delay self heterodyne method are carried out and an experimental system is built to verify it.The experimental results are analyzed and the error of the measurement results is simulated and analyzed.The specific reasons and limiting factors that the traditional method can not achieve the linewidth measurement below 1kHz are clarified.Thirdly,aiming at the increasing demand of linewidth measurement below 1kHz,this paper presents an improved two parameter linewidth measurement method based on partially coherent optical interference.An experimental system is built to verify it.The error of the experimental results is analyzed.and system advantages are summarized.