Adjustment Method and Strain Sensing Characteristics of Whispering Gallery Mode Resonance in Hollow Microbottle
Objective The whispering gallery mode resonator(WGMR)has a very high quality factor and a very small mode volume,so it has great advantages in the fields of laser,optical communication,and biomedical detection.Because different applications may have different requirements for resonance peak parameters,scholars have proposed a series of resonance peak tuning methods.The tuning parameters include the wavelength of the resonance peak,Q value,coupling efficiency,etc.The resonant wavelength is directly related to the refractive index and size of the resonant cavity.In addition,the mechanical tuning method is the most simple and feasible method to control the WGM resonant wavelength in a wide range.In this paper,a double-handle hollow microbottle is proposed to achieve a simple and feasible wide-range tuning of WGM.The double-handle hollow microbottle is fabricated by using single-mode fiber,and the excited whispering gallery mode propagates in the thin wall of the hollow microbottle.By controlling the axial tensile strain,the diameter of the microcavity and the thickness of the microcavity wall can be adjusted,thereby realizing the tuning of the resonant wavelength and the quality factor Q of the whispering gallery mode.Methods First,the influence of controlling the temperature and stress on the resonant peak wavelength is analyzed theoretically.For the selected l-order WGM,if the environment temperature is changed,the relative variation of the resonant wavelength is determined by the effective refractive index and the microcavity radius.The stress regulation method is to apply an external force to the resonant cavity to deform the cavity,and the refractive index of the microcavity is affected by the photoelastic effect.The quality factor Q is related to the light field energy stored in the resonant cavity and the loss energy per cycle.When the microcavity is stretched,it will affect the radiation loss and coupling loss,thereby regulating the Q value.Second,in terms of the preparation of the structure,two sections of flat Er-doped fiber are vertically inserted into the etching solution for corrosion.Then,the two sections of Er-doped fiber after corrosion are placed on the motors on both sides of the fiber fusion splicer,and the two sections of Er-doped fiber with the concave surface are spliced together by arc discharge to obtain a hollow microbottle.On the basis of the hollow microbottle,the asymmetric hollow microbottle is obtained by using the welding machine to pull the cone on the side of the microcavity.Third,the tapered fiber is fixed,and the microcavity is constrained to be perpendicular to the tapered fiber.The two three-dimensional displacement stages hold the fiber at both ends of the microbottle cavity,which can accurately adjust the coupling distance between the tapered fiber and the microbottle cavity and can control the stretching amount of the fiber at both ends of the micro-cavity,thereby applying axial stress.The whole coupling process is completed with the assistance of a high-definition electron microscope,which is used to observe the coupling state of the composite microcavity and the tapered fiber.The tuning results are observed by spectral changes.Results and Discussions Under the action of strain,the axial elongation of the hollow microbottle leads to a decrease in the radius and refractive index of the resonant cavity,which makes the resonant peak move to the short wavelength,and it is consistent with the description of the resonant wavelength change in Eq.(3).Figure 4(b)gives the relationship curve between the resonance peak and the strain change.The wavelength tuning efficiency of the resonance peak is 0.482 pm/με;the wavelength tuning range of the resonance peak is 0.4 nm,and the linearity can reach 0.999.By applying strain to the resonant cavity,the wavelength corresponding to a certain resonant mode can be adjusted to the expected value.It can be seen from Fig.5 that when the strain applied to the microbottle is gradually increased,the Q value of the resonance peak at 1548.92 nm increases first and then decreases during the process of moving to 1548.83 nm.According to the analysis in 2.2 section,it can be seen that this is related to the change of coupling loss.The tuning range of the asymmetric hollow microbottle cavity structure is increased to 0.66 nm(Fig.7).We put the tuning sensitivity of the two structures into a figure for comparison,as shown in Fig.7(b).By means of asymmetric tapering,the tuning sensitivity of the external strain to the WGM resonance peak increases to 0.795 pm/με,and the linearity reaches 0.999.If it is used as a strain sensor,the Q value of the resonance peak can reach 7.218×104,and the strain sensing resolution can reach 25 με.Conclusions In this paper,a quartz fiber double-handle hollow microbottle prepared by the etching-fusion method is designed.The refractive index and diameter of the microcavity are changed by the axial stretching method,so as to tune the resonant wavelength and quality factor Q of whispering gallery mode.The tuning efficiency is 0.482 pm/με,and the tuning range is 0.4 nm.On this basis,the microcavity structure is improved by asymmetric tapering so that the physical parameters of the microcavity are more sensitive to axial strain.The tuning range of the resonant wavelength reaches 0.66 nm,and the tuning efficiency is increased to 0.795 pm/με.At the same time,the whispering gallery mode tuning method proves that the microcavity has strain sensing ability;the resolution is less than 25 με,and the linearity is 0.999,which provides a new idea for high-resolution strain sensing.
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