Microcavity Characterization By Optical Frequency Domain Reflectometry

Optical microcavities,which can restrict the optical power in a very small volume,play a very important role in the modern optics.It is necessary to extract the parameters of the microcavities accurately and conveniently,such as the loss and the coupling conditions,because the quality of the microcavities will directly affect the performance of the optical devices.At present,optical microcavities are developing in the direction of miniaturization and high-Q value.However,conventional characterization methods are difficult to keep up with the pace of microcavities' development.Firstly,the frequency spectrum extracted by traditional characterization method could be disturbed easily by various factors inside and outside the microcavity.Secondly,it will be very difficult to characterize the microcavity with ultra-high Q value,because the sampling density of the transmitted spectrum must be ultra-high,which poses a challenge to the resolution of the optical spectrum analyzer.Moreover,it is very inconvenient to predetermine the coupling condition of the microcavity when measuring the intrinsic Q-factor.Finally,it is difficult to extract the coupling strength of the microcavity only by means of the transmission spectrum.As a distributed backscattering technology,optical frequency domain reflectometry can display the light propagation characteristics in a specific region in the range domain and measure the spectral response of the device in the frequency domain.In this work,the main purpose is to develop a more reliable microcavity characterization technology,with optical frequency domain reflectometry technology as the research object.Combined with the theory,system construction and the experimental verification,the backscattered characterization of the optical microcavity was studied and the feasibility was verified.The main achievements of this work are as follows:1.We optimized and improved the existing optical frequency domain reflectometry system.We effectively reduced the system noise,improved the measurement sensitivity,and improved the polarization ability of the measurement system.Through the integrated instrumental design,the system is simple,stable,portable and easy to be popularized,so that it is suitable for the characterization of the optical microcavity devices under different types and coupling environments.2.Using the optical frequency domain reflectometry system,we measured the load Qfactor and intrinsic Q-factor of the optical microcavities.By receiving the backwardreflected light of the microcavities,the load Q-factors of the microcavity devices were obtained accurately and conveniently.Furthermore,we proposed a theoretical model to obtain the intrinsic Q-factor of microcavities,and the validity and accuracy of this method are proved by the experiments.3.In view of the characteristics of high Q-factor and multi-modes of the whispering gallery mode microcavity,we proposed a new representation method of the Q-factor.The intrinsic Q-factor of the whispering gallery mode microcavity can be measured accurately and conveniently only by selecting and characterizing two adjacent resonant modes,and there is no need to change the coupling gap of the microcavity for many times.4.We also characterized the coupling parameters of the micro-ring resonators,and proposed a new method to characterize the coupling strength of the micro-ring.The experimental results show that our coupling strength measurement results are highly consistent with the software simulation results.By analyzing the back reflection signal curve of each microcavity,we can intuitively judge the coupling condition of the microcavity.We conclude that the optical frequency domain reflectometry system can be used to extract the loss and coupling parameters of the optical microcavity conveniently,quickly and accurately.Compared with the traditional characterization methods,this technique also has the following advantages: First,there is no need to judge the coupling condition of the microcavity when calculating the intrinsic Q-factor,which makes the measurement more convenient;Secondly,the measurement results are more stable when measuring polarization-sensitive microcavity devices.Thirdly,when the optical frequency domain reflectometry measures the microcavity with the ultra-high Q-factor,the more points will be available for the sampling,so that the detection will be easier.Fourthly,all measurements can be made through the single-ended incidence,which will save the cost of packaging and facilitate the measurements of microcavities which are difficult to be coupled.