超快太赫兹时域光谱系统是基于高速异步光学采样原理进行工作的, 该系统使用2个重复频率可在1 GHz附近变化的飞秒振荡器, 并使用高带宽反馈电路控制其重复频率。2个飞秒振荡器的重复频率存在Δf的失谐, 一个飞秒振荡器的重复频率是1 GHz+Δf Hz, 为泵浦脉冲; 另一个飞秒振荡器的重复频率是1 GHz, 为探测脉冲, 由此提供泵浦脉冲和探针脉冲的时间差, 时间延迟呈周期性变化, 其扫描周期可以由1/Δf给出。此系统摒弃了传统THz-TDS系统所必需的机械延迟线, 采用双光子探测器来产生触发信号。当设定Δf=1 kHz时, 1 ms就可以探测出1个THz谱, 用时10.3 s即可得到动态范围为21 dB、频谱分辨率为5 GHz的太赫兹信号。该系统具有检测速度快和频谱分辨率高的优点, 在需要快速测量的应用环境中有着传统太赫兹时域光谱系统不可比拟的优势。

Ultra-fast terahertz time-domain spectroscopy(THz-TDS) system operates based on the principle of high-speed asynchronous optical sampling. The system uses two femtosecond oscillators whose repetition frequencies can vary around 1 GHz, and uses a high-bandwidth feedback circuit to control the repetition frequencies. There is a difference of Δf in the repetition frequency between two femtosecond oscillators. One femtosecond oscillator has a repetition frequency of 1G+Δf Hz, which is the pump pulse; the other has a repetition frequency of 1G Hz, which provides the time difference between the pump pulse and the probe pulse, and the time delay varies periodically with the scanning cycle. The period can be given by 1/Δf. This system abandons the mechanical delay line necessary for the traditional THz-TDS system and uses double-photon detector to generate trigger signal. When the frequency difference between the two femtosecond laser oscillators is 1 kHz, a THz spectrum can be detected in only 1 ms, and a THz signal with a dynamic range of 21 dB and a spectral resolution of 5 GHz can be obtained in 10.3 s. Hence it can be characterized by fast detection speed and high resolution. It has incomparable advantages over the traditional THz-TDS system in the application environment that requires rapid measurement.