Real-time absolute frequency measurement of continuous-wave terahertz radiation based on dual terahertz combs of photocarriers with different frequency spacings

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.

Measurement of absolute frequency of continuous-wave terahertz radiation in real time using a free-running, dual-wavelength mode-locked, erbium-doped fibre laser

“…The precision of 1.2 × 10 −9 was achieved at a measurement rate of 100 Hz in the present setup; however, it was 100-times worse than that of the previous experiment with two independent free-running mode-locked lasers 10 . In the instantaneous-frequency-calculation algorithm 8 , the precision is largely influenced by the signal-to-noise ratio (SNR) of the beat signals with f beat1 and f beat2 10 . The beat signals measured by LT-InGaAs/InAlAs PCAs in the present setup showed the much lower SNR than the signals measured by LT-GaAs PCAs in the previous setup due to high dark-current noise in the LT-InGaAs/InAlAs PCAs (not shown).…”

“…Figure 4 (a) shows the fluctuations with respect to different gate times. Due to the free-running operation without active frequency control, the fluctuations of f rep1 and f rep2 did not decrease over a gate time of 0.1 s. However, these fluctuations were comparable to those of other commercialized, free-running single-wavelength lasers 10 11 ; this is clear evidence that the two mode-locked operations at λ 1 and λ 2 do not compete with each other and are completely independent of each other. Figure 4 (b) shows the temporal fluctuations of f rep1 and f rep2 , where their frequency deviations from the initial values are indicated by δf rep1 and δf rep2 .…”

“…One promising method of achieving this is the scheme based on photoconductive mixing of CW-THz radiation with a THz frequency comb of a photo-carrier (PC-THz comb) in a photoconductive antenna (PCA) 6 7 8 9 10 11 . In this scheme, the absolute frequency f THz of CW-THz radiation can be determined from a PC-THz comb mode m nearest in frequency to f THz , the frequency interval f rep of the PC-THz comb, and the beat frequency f beat between the CW-THz radiation and the m -th comb mode.…”

“…Therefore, f THz could not be determined in real time. Recently, dual PC-THz combs with different f rep have been used to achieve the real-time determination of f THz based on simultaneous measurement of two f rep values and their corresponding f beat values 10 . In that study, by using dual stabilized or free-running mode-locked lasers with different f rep for the generation of dual PC-THz combs, f THz was determined precisely at a measurement rate of 100 Hz.…”

High precision frequency measurement of terahertz waves using optical combs from a Mach–Zehnder-modulator-based flat comb generator

“…Recently, the detection of terahertz frequency using modelocked solid-state or fiber lasers has been demonstrated, and promising capabilities in frequency metrology have been revealed [7][8][9][10][11]. In this method, it is impossible to measure the frequency of terahertz waves that have a spectral width exceeding half the repetition frequency (typically tens of megahertz in mode-locked lasers), so that a higher repetition frequency is desirable.…”