Development of the data acquisition system for terahertz spectrometer

Abstract

In most Terahertz time-domain spectrometer (THz-TDS) experiments, the lock-in amplifier works with trans-impedance pre-amplifier to amplitude the terahertz pulse accepted from detector. This paper discusses the development of data acquisition system for the transmission THz-TDS. In this system, the cross-correlation software algorithm in SR830 lock-in amplifier from Stanford Research Systems, that is usually used in THz-TDS, has been replaced by parallel hardware algorithm of Field Programmable Gate Array (FPGA) chip with the parallel processing ability. This chip has a faster processing speed and higher accuracy than others. A 24 bit Delta-Sigma Analog Digital (AD) was used in place of the 16 bit successive approximation ADC of SR830. The new AD convertor can reduce the complexity of trans-impedance pre-amplifier circuit and replace the SR555 current amplifiers that designed to work with SRS lock-in amplifiers. Besides trans-impedance pre-amplifier circuit, all function circuits, such as low-pass digital filter, phase-locked loop, Direct Digital Synthesis (DDS) reference source and the core algorithms, are integrated in a FPGA chip, which make the new designed lock-in amplifier with a small volume reduce a dozen times SR830 size.