Design Of High-speed Data Acquisition System For Terahertz Optical Comb Spectroscopy

Terahertz waves are electromagnetic waves between 100 GHz and 20 THz.This band is known as the "terahertz gap" due to a lack of technology and slow progress.With the development of ultrafast optics and semiconductor technology,most of the technical difficulties related to the generation and detection of terahertz waves have been solved,thus the terahertz technology has been rapidly developed and widely used.Unlike Fourier Transform spectroscopy(FTS),the terahertz time-domain spectroscopy system does not use the Kramers – Kr(?)nig relation to obtain the magnitude and phase of the complex dielectric constant of the measured substance,and it has a very high signal-to-noise ratio.Because the response time of current photodetector is much smaller than the pulse width of terahertz pulse,the equivalent asynchronous sampling method is normally used.Traditional terahertz time-domain spectroscopy systems often use optical delay lines for scanning while THZ-TDS systems employ optical asynchronous optical sampling(ASOPS)method as an alternative to optical delay lines to obtain a higher scanning speed,measurement accuracy and time resolution.The THZ-TDS system based on ASOPS and photoconductive antenna(PCA)produces THz optical comb as a result of the optical rectification effect of PCA which eliminates the carrier envelope phase shift(CEP).Such THZ-TDS system in fact is a terahertz double comb spectrum system(THZ-DCS).THz-DCS system has a higher spectral resolution achieving comb-tooth-resolved spectra and thus sets a higher request for RF data sampling.Data acquisition system applied in THZ-TDS device cannot satisfy the needs of higher resolution and larger data sampling size of THZ-DCS system.The data acquisition system applied in the existing THz-TDS cannot meet the sampling requirements of higher resolution and larger data size of THz-DCS system.Therefore,according to the characteristics of THZ-DCS system,this paper introduces a data acquisition system for terahertz optical comb spectroscopy.The main work is as follows:Participating in constructing the optical path of transmission optical comb spectrum system.The hardware and software of optical comb spectral data acquisition system are designed according to the requirements of experiment and application scenery.The data acquisition system includes hardware and software parts which are designed and tested separately.For the experimental consideration,single-period mode and multi-period mode is achieved.The upper computer interface of the software is designed,which is closely coordinated with the function of the acquisition system and allows users to modify the key parameters of the system at any time.The system is tested to measure the absorption peaks of water molecules.By comparing with the HITRAN database,a good result is achieved,which verifies the correctness of the system.