Design And Application Of Continuous-wave Terahertz Imaging System Based On Photomixing

The spectral information of terahertz time-domain spectroscopy is abundant.It can not only identify the components of the objects but also reconfigurable the refractive index and thickness distribution of the objects.But the system is complex,high cost and the speed of imaging is slower.Relatively terahertz pulse wave imaging system,the imaging system of terahertz continuous wave can reduces the complexity of the system,lower the cost and improve the speed of imaging effectively.But the traditional of transmission or reflection system can only obtain amplitude information but can't the phase information,for example,Backward-wave Oscillator or Gun diode and so on.And then the refractive index and thickness of the object information can not be reflected.As both the amplitude and phase information can be obtained simultaneously through the continuous-wave terahertz imaging system based on photomixing.And then,the domestic research is relatively fewer at present.A continuous-wave terahertz imaging system based on photomixing is established in this paper.And the main points are as follows:(1)A continuous-wave terahertz transmission imaging system based on photomixing is established by using two external cavity laser diodes of frequency offset in the THz wave frequency range.And the performance of the system is tested.As both the amplitude and phase information can be obtained simultaneously through the continuous-wave terahertz imaging system based on photomixing.The signal-to-noise ratio(SNR)of the system is 68 d B at the frequency 0.47 THz.The resolution of the system is 1 mm.The imaging use of amplitude information is applied.Such as the imaging of blades,the imaging of perspective and the testing of nondestructive and so on.According to the actual of need to match the experimental of parameters to achieve the optimal imaging results,for example the signal modulation frequency?the time constant of phase-locked amplifier?the sampling rate and the sample of two-dimensional translation electrode speed and so on.(2)The thickness and refractive index of the sample are measured by the phase of information of the continuous-wave terahertz transmission imaging system.Further more,the influences of both transmission intensity and the interference of plane sample on phase measurement accuracy are analyzed.The results show that,the accuracy of phase information is better than 2.0% when the transmission coefficient is higher than 0.5.With reduce of THz transmission,the accuracy is also declined.The higher refractive index of the plane sample,the greater interference influence on phase information measurement becomes.A nd then,it should select the appropriate reference sample for the test samples for testing.