Abstract
Real-time measurement of the absolute frequency of continuous-wave terahertz (CW-THz) radiation is required for characterization and frequency calibration of practical CW-THz sources. We proposed a method for real-time monitoring of the absolute frequency of CW-THz radiation involving temporally parallel, i.e., simultaneous, measurement of two pairs of beat frequencies and laser repetition frequencies based on dual THz combs of photocarriers (PC-THz combs) with different frequency spacings. To demonstrate the method, THz-comb-referenced spectrum analyzers were constructed with a dual configuration based on dual femtosecond lasers. Regardless of the presence or absence of frequency control in the PC-THz combs, a frequency precision of 10(-11) was achieved at a measurement rate of 100 Hz. Furthermore, large fluctuation of the CW-THz frequencies, crossing several modes of the PC-THz combs, was correctly monitored in real time. The proposed method will be a powerful tool for the research and development of practical CW-THz sources, and other applications.
Highlights
When femtosecond mode-locked laser light is incident onto a photoconductive antenna (PCA) for detecting terahertz (THz) waves, sub-picosecond photoconductive switching is repeated in the PCA in synchronization with the laser pulses
Experimental setup The THz-comb-referenced spectrum analyzer that we developed was composed of femtosecond lasers, a PCA for THz detection, and data acquisition electronics
Two of these THz-comb-referenced spectrum analyzers were constructed with a dual configuration based on dual femtosecond lasers, and these were effectively used to determine the absolute frequency of a continuous-wave THz (CW-THz) wave in real time
Summary
When femtosecond mode-locked laser light is incident onto a photoconductive antenna (PCA) for detecting terahertz (THz) waves, sub-picosecond photoconductive switching is repeated in the PCA in synchronization with the laser pulses. The sequence of switching operations is essentially copies of the same switching operation separated by an interval equal to the laser repetition period This highly stable, switching pulse train in the time domain can be synthesized by a series of frequency spikes of photocarrier generation regularly separated by the laser repetition frequency in the frequency domain [1]. This structure is referred to as a THz frequency comb of photocarriers, or a PC-THz comb. The generation of a frequency standard signal has been achieved by using a PC-THz comb in combination with frequency control of the CW-THz sources [15, 16]
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