A Photonic Terahertz Continuous-wave-based Phase Imaging System

THz radiation usually refers to the electromagnetic wave radiation with frequency in the range of 0.1THz-10 THz,which lies between the microwave and infrared light in the electromagnetic spectrum.Compared with X-ray,terahertz radiation does not destroy the detected object due to ionization,so it can be used for detecting living biological tissue and other objects.In addition,terahertz waves feature a strong penetration to many non-polar materials(such as dielectric materials,plastics,cartons,cloth and other packaging materials),and it can detect and perceive the relevant information of an object by tracking the phase change of the object passing through.Because of these unique characteristics,terahertz radiation has wide potential applications in the fields of imaging and nondestructive detection.Terahertz imaging systems can be often divided into electronic and photonic terahertz imaging systems.The terahertz source in an electronic imaging system is typically generated in a way of all-electronic,which has a simple structure and high integration.However the phase noise of the generated terahertz signal is relatively large,and the signal-to-noise ratio is affected by a frequency multiplier employed.Photonic terahertz imaging systems are usually based on heterodyne photo-generated terahertz wave schemes.The terahertz signals generated by this scheme have higher signal-to-noise ratio and higher terahertz phase imaging accuracy.However,the phase noise of the laser source in the photonic terahertz imaging system still has a severe effect on the performance of terahertz imaging.In this work,we propose a self-mixing detection vector imaging system based on photonics,which can eliminate the influence of phase noise and measure the thickness of several samples.In this system,the terahertz signal is generated by microwave photonic technology,received and detected by a Schottky barrier diode detector,and the terahertz phase noise generated by the beat frequency of free running laser can be eliminated by self-mixing reception.In the verification experiment,the dual-band modulation with carrier-suppression was used to generate a two-tone optical signal,and a terahertz signal with a frequency of 300 GHz was generated under the photomixing at a uni-traveling carrier photodiode.Based on the phase information obtained by a lock-in amplifier,several samples with different thickness are imaged and measured.The measured thickness error is about 3.84%.The noise of the terahertz imaging system mainly comes from the laser and the uni-traveling carrier photodiode.Therefore,a mean filter and a median filter are then used to denoise the terahertz image,and image enhancement methods with gray-scale transformation,frequency-domain and morphological are also used to enhance the quality of terahertz image.