High Linearity And Large Range Time Stretch Optical Coherence Tomography

Low-coherence interference based Optical Coherence Tomography(OCT)can obtain non-invasive,high-resolution,and fast tomography in the depth direction.As so far,it has been widely used in medical imaging and industrial measurement.With the promotion of OCT in surgical guidance and measurement of large-size samples,it is extremely urgent to further increase the imaging speed and imaging depth.To achieve this purpose,time stretch based sweep source OCT(TS-OCT)has been introduced,with a A-Scan rate of ～MHz and imaging depth of ～cm.However,due to the nonlinear chirp of the sweep source,the TSOCT requires post-data calibration,which affects the real-time imaging rate.Meanwhile,due to the ultra-fast scanning rate of the light source,when the imaging depth reaches the centimeter level,a detection bandwidth of ～GHz is required.This limits the imaging depth of the TS-OCT and affects its application in large depth imaging and industrial inspection.This thesis explores some solutions from the optimizing the chirp characteristics of the time-stretch light source,reducing the detection bandwidth requirement for large depth imaging,and reducing the time-stretch light source scanning rate,to improve the real-time imaging rate and imaging depth of TS-OCT.In brief,the research achievements and contributions of this thesis include:(1)Basic theoretical model of the TS-OCT is bulit.First,the reasons leading to the nonlinear chirp and ultra-fast sweep rate of the time-stretch light source are systematically investigated.Then the optical high-order dispersion compensation,optical depth segmenting,and dispersion multiplication schemes are theoretically proposed to improve the real-time imaging rate and imaging depth of TS-OCT.Meanwhile,the effectiveness of these schemes is verified by simulation.(2)Calibration-free TS-OCT is demonstrated based on two different optical higher order dispersion compensation schemes,which substitutes the digital calibration by higher order dispersion compensation.As a result,the imaging rate is greatly improved by 82% in the calibration-free TS-OCT.Meanwhile,because of avoiding the noise introduced in the process of digita calibration,the sensitivity roll-off and the imaging quality of large depth are also increased.(3)Large range TS-OCT with meter level imaging depth is introduced based on optical depth segmenting scheme.With the dual-optical frequency comb asynchronous sampling,a large depth range can be cut into multiple small depths for measurement.As a result,the imaging depth of the TS-OCT can be increased from 3 mm to 1.5 m,while having the singleshot A-scan rate of 200 k Hz.Then,the feasibility of extending the imaging depth of the TSOCT to the kilometer level is demonstrated under the premise of the same detection bandwidth.(4)The dispersion multiplication based meter-level TS-OCT is also proposed,which has a small detection bandwidth and a MHz A-Scan rate.With the cross-phase modulation,the dispersion of the fiber can be amplified from 2.59 ns/nm to 287.5 ns/nm.The TS-OCT based on dispersion multiplication can achieve an image depth of 1.5 m and an A-Scan rate of 10.24 MHz under 6-GHz detection bandwidth.