Research On Photonics-based Terahertz Amplitude Imaging Techniques

Terahertz waves(THz)are located between the microwave and infrared bands with wavelengths ranging from 3 mm to 0.03 mm(0.1 THz-10 THz).Because the terahertz waves are penetrative to non-polar materials and dielectric materials,and explosives,chemicals,and biological agents have unique reflection spectra in the terahertz range,so terahertz waves have broad application prospects in the imaging field.In the past decades,with the fast development of THz devices,THz imaging technology has progressed rapidly.In the terahertz imaging systems,one method of generating THz is based on the electronic method through microwave frequency multiplication.However,this method has some limitations in bandwidth,serious distortion and phase noise.In contrast,the photo-electronic method performs THz generation based on frequency beating in a uni-traveling-carrier photodiode(UTC-PD)through two optical carriers The photo-electronic method avoids frequency multiplication processing,and no harmonics,therefore features larger bandwidth,smaller phase noise interference and more flexibleTypically,photonic terahertz imaging systems can be divided into scalar imaging and vector imaging systems.The scalar systems process only the amplitude information,and the vector systems measure both the amplitude and phase information of samples However,due to the instability of the light source,the terahertz imaging system causes terahertz frequency to drift,and the signal quality is degraded at the receiving end due to the unstable terahertz frequency,thereby affecting the imaging qualityIn response to the above problems,this paper proposes a design scheme based on photo-generated terahertz double-band modulation amplitude imaging system,which transfers the sample information in the modulation signal,which can eliminate the impact of imaging quality because of terahertz frequency drifting.On the basis of double-sideband modulation,a non-suppressed double-sideband modulation imaging scheme is proposed,which can increase the received signal powerThe thesis first introduces the key components and working principles of the photo-generated terahertz amplitude imaging system in this design,including electro-optic modulators,terahertz photodetectors,and terahertz receivers.The module in this system includes optical system design,optical path design,receiver circuit design,and software design.We theoretically derivatives that the received terahertz power by employing non-carrier suppressed electro-optic modulation is higher than the carrier-suppression modulation.We theoretically derivatives that the received terahertz power by employing non-carrier suppressed electro-optic modulation is higher than the carrier-suppression modulation.We also conduct a terahertz amplitude imaging experiment with a carrier frequency of 300 GHz and modulation frequency of 12 GHz,and the 3dB higher received power is validated.Moreover,the experimental results indicate the estimated imaging error of measuring sample thickness is 0.46 mm by using the proposed technologyThis work expects to provide a technical solution for terahertz continuous-wave imaging with wideband frequency tunability.