| Dual optical comb ranging(DOCR)is a novel laser measurement technique using cross-correlation sampling of femtosecond,and it has the capabilities of large-range,high-precision,and online traceable absolute ranging.Compared to the conventional time-of-flight LIDAR,DOCR with micron measurement accuracy has more stringent requirements on the beam quality of echo pulse,and the normal transceiver with independent emitting and receiving modules is not applicable for DOCR system.On the basis of the application requirements of DOCR for long-distance measurement,and the working principle of DOCR and the beam shaping method of transmissive optical system,a newly-designed transceiver integrated optical system structure is proposed and demonstrated for laser ranging.The reported system has the capabilities of effectively reducing the light propagation loss and increasing the beam echo energy and improve the beam quality.The main work is as follows:Firstly,the entire scheme of optical system design is presented.According to the idea of modularization,the optical system for transmitting and receiving is split into three modules: collimation,beam expansion and focusing,in which the laser transmitting and receiving share the expanding/shrinking module.The utilization of coaxial optical transceiver assistants reducing the blind area of receiving field and improve the visibility of interference.The causes of light energy loss in the ranging process are analyzed,and the design indexes of transmitting and receiving optical system are established.Furthermore,the design of collimating,beam expanding and focusing modules is completed,and the overall performance analysis is carried out.The beam diameter of the collimating module is Φ3.4 mm,and the divergence angle reaches 0.56 mrad of the current diffraction limit.The beam expander module adopts three kinds of constant multiplication structures with different magnification,and the RMS wavefront difference is less than 0.03 λ,which can be applied to different surveying occasions.In order to enhance the flexibility of the system,a continuous adjustable beam expanding lens of 2～3.5 times is designed in terms of the continuous zoom theory.Under the unfavorable condition of the diaphragm moving forward,the f-number of the focusing module is 2.1,and the beam within the field of view of 0.37° can be incorporated into the receiving target of only 0.2 mm.The diameter of laser emission,divergence angle and Rayleigh range are Φ10 mm,0.106 mrad and 43.55 m,respectively.The mechanical structure design of the ranging system is performed,and the stray light of the system is obviously suppressed through the simulation of non-sequence components.The aberration distribution of the system is reasonable,and the tolerance setting is loose and easy to be machined.Finally,the DOCR experiment is accomplished to verify the beam shaping ability of the proposed optical system.When detecting the cooperative target at 20 m,the echo energy loss is only 4% compared with loss data at 0 m distance.The range standard deviation of 20 μm can be obtained by moving average in non-fuzzy range(2.12 m),and the precision and dependability of the ranging are verified by the interferometer.The experimental data illustrates the transceiver system has not impacts on the ranging precision of DOCR.With the purpose of expanding the measurement range of existing DOCR system,a newly-designed transceiver optical modality with compact structure and strong reliability is developed,which potentially enable the performance of DOCR in various practical LIDAR applications. |
