Decade-Spanning High-Precision Terahertz Frequency Comb

The generation and detection of a decade-spanning terahertz (THz) frequency comb is reported using two Ti:sapphire femtosecond laser oscillators and asynchronous optical sampling THz time-domain spectroscopy. The comb extends from 0.15 to 2.4 THz, with a tooth spacing of 80 MHz, a linewidth of 3.7 kHz, and a fractional precision of $1.8\times {10}^{-9}$. With time-domain detection of the comb, we measure three transitions of water vapor at 10 mTorr between 1–2 THz with an average Doppler-limited fractional accuracy of $6.1\times {10}^{-8}$. Significant improvements in bandwidth, resolution, and sensitivity are possible with existing technologies.

Figure 1

The experimental layout for the generation and detection of the THz frequency comb. See text for details.

Figure 2

(a) Fourier transform of a 225 ns THz pulse train in delay time yields a decade-spanning frequency comb that extends up to 2.4 THz, with a 4.4 MHz tooth width. The peak dynamic range of the comb is 30 dB after 30 min of integration. (b) The tooth width of the THz comb can be reduced to 3.7 kHz by extending the time-domain measurement to $268\mu \sec$ in delay time. The peak dynamic range is 20 dB after a single 107 s acquisition. Precision is preserved over the entire bandwidth of the comb, as shown in the bottom two panels.

Figure 3

(a) The broadband spectrum of water vapor at 100 mTorr with 11 repetition rate steps. The predicted spectrum from the JPL spectral line catalog [21] is shown in red, with negative intensity. The asterisks indicate saturated transitions. (b)–(d) The Gaussian fit (red line) to three transitions of water vapor at 10 mTorr. Line centers from the JPL catalog are indicated with vertical blue dashed lines.